I think you mean (x, 8) lies on the line (disable smilies! :) ). If you can find the equation of the line, you can plug in y=8, solve for x, and you're done. We can find the equation of the line in two ways:
find the slope, using (y_2 - y_1)/(x_2 - x_1), then plug in a point to find the y-intercept.
or, plug both points into the equation y=mx + b, and solve for m and b (this is normally fastest, so that's what I'll do):
-7 = -5m + b
- -4 = 3m + b
-3 = -8m
m = 3/8
b = -41/8
So y = (3/8)x - 41/8
Plug in y=8:
8 = (3/8)x - 41/8
64 = 3x - 41
105 = 3x
35 = x
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