Sum of a number and 7 is subtracted from 15 the result is 6. The phrase a number means an arbitrary variable, let's call it x. We take this expression in pieces. Sum of a number and 7 x + 7 Subtracted from 15 15 - (x + 7) The result is means an equation, so we set this expression above equal to 6 15 - (x + 7) = 6 <-- This is our algebraic expression If the problem asks you to solve for x, we Group like terms 15 - x - 7 = 6 8 - x = 6 Type 8 - x = 6 into the search engine, and we get x = 2