To model -3x + 7 = 28, you have to find the additive inverse of 7. Since -7 is the additive inverse of 7, you subtract both sides by -7 and you bring down -3x and the = sign. Make sure you draw a long line beneath the equation. Cross out the -7 and 7. So 28 - 7 is 21. Next is to find the multiplicative inverse of -3x. So -3x = 21 is -7. Before that, you divide both sides by 3 and cancel out the -3x/3. Bring down x and = and your answer is -7. x = -7