Help!! 20 Points! Being Timed !

Average acceleration: i have no idea how to do this.

Which expression do you get when you eliminate the negative exponents of 4a^2 b^216a^-3 b

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