Solve the simultaneous equations. 5x + y = 21 and x-3y = 9

Label the equations, [a] 5x + y =21 and [b] x-3y =9. [b]: x - 3y = 9, [c]: x = 3y + 9. Substitute [c] into [a] 5(3y + 9) + y = 21 , 16y + 45 = 21 16y = -24 , y = - 3/2. Substitute y = -3/2 into [b]: x + 9/2 = 9 , x = 9/2. Check answers: 5(9/2) + (-3/2) = 21 Yes , 9/2 - (-3/2) = 9 Yes.