A simplified model for the concentration $($micrograms$/$milliliter$)$ of a certain slowreacting antibiotic in the bloodstream $t$ hours after injection into muscle tissue is given by

$f(t)=t^2*e^{-\frac{t}{18}},t0$

$($a$)$ When will there be maximum concentration?

$($b$)$ In the units given, how much is the maximum concentration?

$($c$)$ When will the concentration dip below a level of $20\mathrm{.}0?$

$($d$)$ Estimate from a graph when the concentration function changes concavity: