How Do I Solve 2x-3y=9 X=-3?

You can solve for y, then substitute x=-3, or you can substitute x=-3 and solve for y. Let's do it the second way first.
2x -3y = 9    (the original equation)
2(-3) -3y = 9    (equation with -3 substituted for x)
3(-2 -y) = 3*3    (factor out 3)
-2 -y = 3    (divide both sides by 3)
-y = 3 + 2    (add 2 to both sides)
y = -5    (multiply both sides by -1)
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Solve for y ...
2x -3y = 9
2x = 9 + 3y    (add 3y to both sides)
2x -9 = 3y    (subtract 9 from both sides)
(2/3)x - 3 = y    (divide both sides by 3)
(2/3)(-3) -3 = y    (substituted -3 for x)
-2 -3 = y    (perform the multiplication)
-5 = y    (perform the addition)