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 $T(x)=-0\mathrm{.}75x^4+9x^3,0x9.$ 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 for T$-$shirt production? Graph T and $T^{\text{'}}$ on the same coordinate system.N