An object 5 millimeters high is located 15 millimeters in front of a plane mirror. How far from the mirror is the image located?

Question

using the lens equation..

1/f= 1/o + 1/i

here, o is fifteen cm, focal length for a plane mirror is infintity, so

0=1/15 + 1/i solve for i. It should turn out to be -15cm, the negative sign means behind the mirror.

how do i remember phmpoc