The first step is to remember the order of operations, PEMDAS, or Parentheseis, Exponents, Multiplication and Division, and finally Addition and Subtraction.
If this is 21 * (4 * 5) - 23, then here's the steps:
21 * (4 * 5) - 23
Multiply the numbers within the parenthesis:
21 * (4 * 5) - 23
21 * (20) - 23
Multiply:
21 * (20) - 23
420 - 23
And hopefully the last step is easy enough, subtract:
420 - 23 = your answer
