Imagine your eye is at the top of the blue line at a height of $h$.
You want the image to appear to be standing straight up where the
red line is, a distance $d$ horizontally from your eye.
For the yellow point to appear a height $x$ above the ground, it
needs to be drawn a distance $y$ along the ground from the base of
the red line.
If you knew $h$, $d$ and $x$, how could you work out $y$?