An object located 13cm in front of a concave mirror creates a real image 4 cm in front of the mirror. A second object, three times as tall as the first one, is placed in front of the mirror but at an unknown location. The image of this second object is half the height as the first image. How far in front of the mirror is the second object?
(Please explain the steps by using the variables do, di, f, ho, hi)