A circular oil slick is expanding with radius, $r$ in feet, at time $t$ in hours given by $r= 6 t - 0.3 t^2,$ for $t$ in hours, $0 \le t \le 10.$

Find a formula for $A = f(t)$, the area of the oil slick as a function of time.

$A = f(t) =$ help (formulas) with help (units)
(Be sure to include units!)