The sum of five and twice a number is 17 The phrase a number means an arbitrary variable, let's call it x x Twice a number means we multiply x by 2: 2x The sum of five and twice a number means we add 5 to 2x: 2x + 5 The phrase is means an equation, so we set 2x + 5 equal to 17 to get our algebraic expression 2x + 5 = 17 As a bonus, if the problem asks you to solve for x, we type in this algebraic expression into our math engine and we get: x = 6