from dV/dr = 40(pi) - 2r(pi) - 3r^2(pi),
set dV/dr =0,
So, 40(pi) - 2r(pi) - 3r^2(pi)=0.
take out pi as a factor, and solve the remaining quadratic for r.
as the question asks for the positive value of r, choose the appropriate one and sub it back in to the original equation to find V.