Question

For the question:
Given that the determinant of the 3x3 matrix of (left to right, top to bottom) (a, 1, 1)(b, 2, 1)(c, 3, 1) is -9, find the exact value of x when ax+y+z=1,bx+2y+z=-2 and cx+3y+z=3.
I've already figured out from the matrix that -a+2b-c=-9, what should I do now?