Writing relations as a set of ordered pairs?

Which of the fallowing best completes the sequence 12,8,5, a) 1 b) 2 c)4 d)0

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

X+y=-4 x-y=2 use elimination with adding and subtraction