16 decreased by 3 times the sum of 3 and a number Take this algebraic expression in parts: The phrase a number means an arbitrary variable. Let's call it x. The sum of 3 and a number: 3 + x 3 times the sum: 3(3 + x) 16 decreased by... means we subtract 3(3 + x) from 16 3(3 + x) from 16