Using the mirror formula 1/f = 1/u + 1/v. For a convex mirror, f is positive, and the object distance u is negative. 1/f = 1/(-2f) + 1/v => 1/v = 1/f + 1/(2f) = 3/(2f). So, v = 2f/3. Magnification m = -v/u = -(2f/3) / (-2f) = +1/3.