What property is being shown? 5 + (6 + 3) = (5 + 6) + 3

If i have 20$ and i gave 5$ away how much i got

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

What is the axis of symmetry of the function f(x)=-(x+ 9)(x-21)