What is the average value of a function y=3x^5 on the interval [-3,3]?

It is $0$.
The average value is $\frac{1}{3 - \left(- 3\right)} {\int}_{-} {3}^{3} 3 {x}^{5} \mathrm{dx}$
But since ${\int}_{-} {3}^{3} 3 {x}^{5} \mathrm{dx} = 0$, the average value is also $0$.