Why does -7 - (-4) have the same results as -7 + 4? For someone else

-7-(-4) is the same result as -7+4 because any negative added to a negative sign will be a positive. When adding mainly like this it is super easier to automatically change the -(-4) to a positive then just do 7-4 and you will get 3 then just add the negative sign

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