Subtracting in Google Sheets requires a particular syntax, and there are two ways to subtract cells and numbers in Google Sheets. One method can use multiple numbers and cell references. The other technique uses the MINUS function and is limited.
Use the Minus Sign to Subtract in Google Sheets
When you subtract on paper, you use the minus (  ) symbol between the numbers, which is also how Google Sheets does it. The only difference is that you refer to cells instead of numbers.
For example, to subtract two numbers on paper, you'd write something like 4517. Google Sheets can do this, but you reference the cells instead of the values, such as B1C3.
To make this subtraction formula work in Google Sheets, precede the equation with the equal sign ( = ), like this:
=B1C3
As shown in the above example, you need to write the formula needs in the cell that will display the result. This is A1 in the sample image.
You can mix in real numbers if you don't have a cell with that value in it. Here's an example of that:
=150B1C3
As you use the formula bar in Google Sheets, each color in the formula is shown in the corresponding cell references. This helps you keep track of what you're doing, and is useful as you add more parts to create a longer formula.
How to Subtract Using the MINUS Function
Another way to subtract in Google Sheets is to use the subtraction function, called MINUS. The only catch with this method is that it works with only two numbers.
The syntax of this formula is a bit different than when you use the hyphen to subtract. Here's what it would look like to subtract A2 from A1:
=MINUS(A1,A2)
To visualize this formula like the first method above, we could write it like this with the same result:
=A1A2
The order of the terms is important. When the A1 and A2 references in the formula above are switched, the result is 70 since the formula subtracts 120 from 50.
You can also enter a number, either in both number slots or in one, like this:
=MINUS(45,A3)
Remember the Order of Operations
Equations of all types are generally calculated in lefttoright order, with the caveat that certain things are calculated before other things. The precedence of what is calculated first is called the order of operations, and it follows this sequence:
 Parenthesis: In order from the innermost to the outermost parens.
 Exponents: Usually rendered with a caret symbol ( ^ ).
 Multiplication and division: Rendered with * and /, respectively.
 Addition and subtraction: Rendered with + and , respectively.
Take this formula as one example:
=(3+3)*2^2((1+2))+3
This is how Google Sheets deals with all of those numbers:

Takes 3+3 to create 6*2^2((1+2)+3).

Takes 1+2 to simplify as 6*2^2(3+3).

Takes 3+3 to make 6*2^26.

Figures out the value of 2^{2} to simply as 6*46.

Multiplies 6 and 4 to make 246.

Subtracts 6 from 24 to produce 18.
More Complex Formulas
The subtraction formula, because it works with many different terms, can also use cell references instead of numbers, and it can also compute functions as it resolves the math.
There's no practical limit to the terms, values, and functions you can employ within the subtraction formula. For instance, the following formula, despite being unwieldy, is syntactically correct:
=((3+3)^4)*(sum(a1:a6)150*(minus(c3,d45)
This is why the Google Sheets colorcoded cell references help decrypt complex formulas in documents like financial statements and yearend reports.