Mark M. answered 02/02/17
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Retired math prof. Very extensive Precalculus tutoring experience.
g(x) = 3-x2
g(x+h) = 3-(x+h)2 = 3-(x2+2xh+h2) = 3-x2-2xh-h2
So, g(x+h)-g(x) = -2xh-h2
[g(x+h)-g(x)]/h = (-2xh-h2)/h = [h(-2x-h)]/h
= -2x-h