how do i simplify this equation

Anytime you are multiplying a number times something in parenthesis you multiply it by EVERY individual number in the parenthesis. In this case 5(2a+3)+2a+3 is the same as 5(2a)+5(3)+2a+3 or 10a+15+2a+3. You can only combine "like" terms, the a's and the numbers, so the final is 12a+18. The easiest mistake to make when multiplying through parenthesis is getting the sign wrong, as in 5-2(3a-4). the number you are multiplying through the parenthesis is -2 not 2. Easy mistake to make.