What is

3x-2y=9
y=2x-7

Okay, these get easy with practice so don't post all of these online. First, substitute y in 3x-2y=9 for what it says y equals (2x-7). You now have: 3x-2(2x-7)=9. Breakdown:
3x-2(2x-7)=9
3x-4x+14=9
-1x=-5
x=5
next solve for y
y=2x-7
y=2(5)-7
y=10-7
y=3
And now we have x=5, and y=3.

Since you know that "y" is equivalent to "2x - 7" you can substitute that into the first equation

3x - 2y = 9
3x - [2(2x-7)]= 9
3x - (4x - 14) = 9
3x - 4x + 14 = 9
-x + 14 = 9
-x = -5
x = 5

y = 2x - 7 
y = 2(5) - 7
y = 10 - 7
y = 3