I did not understand all of it, what you wrote, but some of it yes.
I wont solve it for you, I will give you hints, ok?
HINTS
1. express your knowns and unknows
- distance to first position
distance to second position = distance to first + 30
...
there you have angle, distance on the ground and you want to know height
if you draw it with right angles you will see it is cosinus
you express cosinus and play with it ...