For what values of the coefficients a and b is (2,-1) the solution to the following linear system?

ax + by = -7

2ax -3by = 1

(Answer: a= -2, b= 3)

I've been taught to find the values of x and y by substitution or elimination, but how do you find the values of the coefficients a and b is (2,-1) as in the above question?

Please help.

Thanks.