Answer:
For a) $A(r(t))=π(4t)^2.$
For b) 803.84
Step-by-step explanation:
For a) we can do a simple substitution on the variable r. Notice that $A=πr^2$ make $A$ a function of $r.$ Then, $A(r(t))=\pi (r(t))^2=\pi (4t)^2.$
For b) you only need to substitute the value $t=4$ on the expresión $A(r(t)).$