$19.($a$)$ A concave spherical mirror forms an inverted W image $4\mathrm{.}00$ times larger than the object. Assuming the distance between object and image is $0\mathrm{.}600$ m $,$ find the focal length of the mirror. $($b$)$ What If$?$ Suppose the mirror is convex. The distance between the image and the object is the same as in part $($a$),$ but the image is $0\mathrm{.}500$ the size of the object. Determine the focal length of the mirror.