Question

How do I find where the function is increasing or decreasing in the function f(x)=-5x^2-90x-18?

Answer 1

find the points where f'(x) >0 and f'(x)<0. The points where it is greater than 0 is increasing and where it is less than 0 is decreasing.

Did you understand this @GabbyVille :)
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Answer 2

f'(x)=-10x-90
=-10(x-9)

so at points (9,infinity) it is increasing and decreasing at (-infinity,9)