Can someone me with question number 9 ?

With #! i’ve tried to figure it out but i can’t.

Solve this system of equations. 12x − 18y = 27 4x − 6y = 10

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