Find the focal length of a concave mirror if an object 17 cm

Find the focal length of a concave mirror if an object 17 cm from the mirror has a virtual image 2.6 times the object\'s actual size.

Express your answer in cm using two significant figures.

magnification m = +2.6

m = -s\'/s

-s\'/s = +2.6

s\' = -2.6*s = -2.6*17

from lens equation

1/s + 1/s = 1/f

1/17 - 1/(17*2.6) = 1/f

f = 27.6 cm