If the equation x + 2Y = - 1,2x-y = 3, kx-y = 9, find the value of K

If the equation x + 2Y = - 1,2x-y = 3, kx-y = 9, find the value of K

By combining the first two formulas, we can get x = 1, y = - 1. The third formula is K + 1 = 9, so k = 8