Since point p is a point that is shared, by both the line, y equals 850, and the
line y, equals 10x plus 150, both of these lines have the same y value at that
point. Since the y value of y equals 850 is always 850, that means that this the
y value of 10x plus 150 at point p. So, we just need to substitute 850 into the
spot of y, in the second equation. That gives us a single equation of 850 equals
10x plus 150. And yes, x is the only variable in this equation.