PEMDAS is the order of operations.
The list below is from highest precedence to lowest precedence.
MD#->#Multiplication & Division from Left to Right
AS#->#Addition & Subtraction from Left to Right
Parenthesis have the high precedence and should be worked from the innermost to the outermost.
Next you would work on any expressions that are raised to a power, exponent.
Next if you have multiplication and division those should be evaluated from the leftmost moving to the right.
Lastly, if you have any addition and subtraction those should be evaluated from the leftmost moving to the right.
This is an agreed upon method resolving or evaluating expressions and equations. Without this agreement people working on mathematics would come to different conclusions based on the operations they chose to evaluate at random.
If you ever come to the point where you want some part of an expression or equation to be evaluated at a higher precedence that you just have to enclose it in parenthesis.