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 $y$?