Image due to the lens would be at 180 cm to the right of it. This means 80 cm to the right of the mirror. This image would behave as virtual object for the mirror and mirror would try to form real image at 80 cm along vertical direction (due to 45 deg. inclination). While mirror would have formed image 80 cm along the vertical had there been air/vacuum, there is water which will bend the rays towards the normal causing diverging effect. This diverging effect would cause shift in the path. To have image at the bottom, we must have:
\((1-\frac {1}{\mu})t=H-80\)
From the above, H can be obtained.