Can someone help me with these questions

Solve: 3x+y=6 4x-2y=18 show all work !

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

If f(x) = 2x + 8 and g(x) = x4, what is (gº)?