Ugg, what the heck is subsitution.. hmm sounds familar, perhaps:

3x+2y=4

y=-2x+3

First solve for x, or y, it doesn't matter, I am solving for x.

3x = 4-2y

x = 4/3 - 2y/3

Now subsitute x for the value I above, and solve for y. Then use the y answer (which should be one number, not an expression) to solve for x.