Bidmas is an order of operation used when doing equations, which include both addition and subtraction, or Multiplication and Division.

This is the order it goes in.

Brackets
Indicies
Division
Multiplication
Addition
Subtraction

Another anagram for this is "bodmas" identical, but the "o" stands for "power of" i.e.

Brackets
power Of
Division
Multiplication
Addition
Subtraction


e.g. When given a simple example:
(56 - X)(Y + 7) where X=6 and Y=3.
You must use bidmas. First solve the brackets, which include addition and subtraction but are exempt from the rule as they are inside the brackets and then multiply the two values.

(56 - 6) x (3 + 7) =
50 x 10 = 500