When is the rate of change of t-shirt production increasing

Question

Production: point of diminishing returns:-

A T-shirt manufacturer is planning to expand its workforce. It estimates that the number of T-shirts produced by hiring x new workers is given by

T(x)=-0.25x⁴ + 5x³            0<=x<=15

When is the rate of change of T-shirt production increasing and when is it decreasing? What is the point of diminishing returns and the maximum rate of change of T-shirt production? Graph T and on the same coordinate system.