Always use your note. Always. If there is some other way to proceed, do that second.

f'(x) -- What is it? Do you have the definition that relates it to "slope"? that is what you need on this one.

Slope = (Change in y)/(Change in x) = (Change in f(x))/(Change in x)

Using the sugested approximation

f'(x) for x = 0 is approximately (f(5) - f(0))/(5 - 0) = (80-110)/(5-0) = -30/5 = -6

Note: It doesn't much mater which way you subtract them, as long as you are consistent.

(110-80)/(0-5) = 30/(-5) = -6 -- Still good.

(110-80)/(5-0) = 30/5 = +6 -- NO good.