Question 164379
Draw a normal triangle, angle from the light to the top is 75, other angle from observer to the top is 45. From the top straight to the bottom is 90 degree, and you got 2 right triangle. The height is h ( as you said ).
call x : adjacent side of 45 degree
515-x : the left over which next to 75 degree.
 Then h= tan 45 (x)  Also h = tan 75 ( 515-x)
That means: tan 45(x)= tan 75(515-x)
Solve x. Then plug x to the equa. tan 45(x) --> then you got h.
Good luck.