A point object is placed at a distance of 60cm from a convex lens...

first, for image formation from lens
$u = - 60cm$

$f = + 30cm$

$⇒ v = \dfrac{uf}{u + f} = \dfrac{- 60 \times 30}{- 60 + 30} = 60cm$

this real image formed by lens acts as virtual object for mirror

Real image from plane mirror is formed $20cm$ in front of mirror, hence at $20cm$ distance from lens. Now, for second refraction from lens, $u = - 20cm$

$f = + 30cm$

$v = \dfrac{uf}{u + f} = \dfrac{- 20 \times 30}{- 20 + 30} = - 60cm$

So, final virtual image is $60cm$ from lens, or $20cm$ behind mirror