I'm stuck on..

a calculus question

f(x) = x^4 - 40 x^3 State the values of x for which f is decreasing. Use +infinity or -infinity for the symbols. and State the values of x for which f is concave down

heres what i have: the values of x when slope = 0 is (0,30) max. and mim. are: none and 30 one of the inflection points is (0,?)

i know the first part should look like when f is decreasing : -infinity

Question

I'm stuck on..

a calculus question

f(x) = x^4 - 40 x^3 State the values of x for which f is decreasing. Use +infinity or -infinity for the symbols. and State the values of x for which f is concave down 

heres what i have: the values of x when slope = 0 is (0,30) max. and mim. are: none and 30 one of the inflection points is (0,?)

i know the first part should look like when f is decreasing : -infinity<x<?