x= (5;6) andy= (-1;0), vectors in R^2. Find the unique basisC= {w1;w2} of R^2 with respect to which [x]C= (-1, 2) and [y]C= (2,-1).

I know how to find [x] and [y] when given x, y and w's, but I dont know how to work backwards to find w1 and w2.