What is the measure of an angle whose complement and supplement equal 200?

The complement is 90-n and the supplement is 180-n.  If I understand your wording properly, we now know that 90-n + (180-n) = 200.

-2n + 90 + 180 = 200.  Solve for n:  270-200 = 2n, so n = 35 degrees.

The slope of the function is 0.5.