A spherical mirror has a focal length of −0.59 m. An object of height 0.045 m is located 0.42 m in front of the mirror. What is the height (including its sign) of the object’s image?

1/F =1/do +1/di

F is the focal length,
do is the distance between the object and the back of the mirror,
di is the distance between the image and the back of the mirror.

di and F are negative since they are both behind a convex mirror.

do = 0.42 m, F = - 0.59 m.
1/F =1/do +1/di
-1/0.59 = 1/0.42 + 1/di
1/di =-(1/0.59 +1/0.42) = - (1.69 +2.38) = - 4.07
di = - 0.245 m (the image is behind the mirror)
M = di/do = hi/ho
hi = ho• di/do = 0.045•(-0.245)/0.42 = - 0.026 m