Please help i can't figure this out

In a graph with several intervals o data how does a constant interval appear? what type of scenario produces a constant interval?

Witch represents the inverse of the function f(x)=1/4x-12

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