In equations of this type where there are variables on each side of the equals, you need to bring the variables to one side and the known values on the other. In this case add 3y to each side of the equation: 0=14y+7. Then subtract 7 from each side: -7=14y. You've now separated the unknowns from the knowns. There's a common factor 7 which you now divide through by: -1=2y. Turn this round because it's an equality: 2y=-1. Finally divide through by 2: y=-1/2.