Simple math question...

That I'm too rusty on the math to work out... I'm trying to help a friend study for the GRE and this is one of his practice test questions. How does one go about deriving the answer?

If 3xy=4 and x-2y=7, what is 3(x^2)y - 6x(y^2)?

a. 3, b. 12, c. 28, d. 42, and e. 60

Look for factors of either 3xy or x-2y, as those are the ones you know.
3*x*x*y=3xy*x
-6*x*y*y=3xy*(-2y)
And then you see you can rewrite to
(3xy * x) - (3xy * 2y), and
that is 3xy *(x-2y)=4*7.

Thanks.