PEMDAS is a commonly used acronym that helps students learn the order of operations. The letters stand for the following:

Parenthesis

Exponents

Multiplication and Division

Addition and Subtraction.

This means that when simplifying a mathematical expression, the student must simply the contents of any parenthesis first, and then the student must perform any given exponentiation.

Next, the student must perform any multiplication or division as indicated (ALWAYS from left to right).

Finally, addition and subtraction must be performed, again, always left to right.

It should be noted that any expression contained in parenthesis acts sort of like its own expression, and follows the rules of PEMDAS, the same way the bigger expression does.