2/17 = x/68 find the proportion

Given the equation f(x)=a(b)x where the initial value is 40 and the growth rate is 20% what is f(4)

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

Me n o w. i need t h i s. 10 points