Optics: A Concave Mirror Forms An Inverted Image Four Times Larger Than Object. Find Focal Length?

Question

A concave mirror forms an inverted image four times larger than the object. Find the focal length of the mirror if the distance between the object and the image is 50 cm.

I guess I use the formula M = h(i)/h(o) = -q / p

and

1/p 1/q = 1/f

Answer 1

-q/p = -4

q - p = 50

isolate q from the first equation and substitute into the second

(4p) - p = 50

3p = 50

p = 50/3

(3/50) + (3/200) = (1/f)

(15/200) = (1/f)

f=13.3333333 cm