What to do when the says of in a problem

Apinhole camera is made out of a light-tight box with a piece of film attached to one side and pinhole on the opposite side. the optimum diameter d(in millimeters) of the pinhole can be modeled by d=1.9[ (5.5 x 10^-4) l) ] ^1/2, where l is the length of the camera box (in millimeters). what is the optimum diameter for a pinhole camera if the camera box has a length of 10 centimeters

What is the solution to the division problem below x3+x2-11x+4/x+4

Given the functions, f(x) = 2x^2 - 1 and g(x) = 5x, find f(g(x)) and g(f(