The order of operation is divided in 4 options:
1st - Breaking dow parenthesis
2nd - Square roots / exponents
3rd - Multiplication / division
4th - Addition / Subtraction
So in your case the expression will look like this:
18 + 4 * 3 = → we solve the multiplication first
18 + 12 = → secondly we solve the addition (this is the simplified operation)
30 → final answer
Hope it helped,
BioTeacher101