Math question - need answer and working - preferably with explanation?

Question

an observer spots 2 planes directly overhead. the angle of elevation from the runway to the planes is 18* and 25*. if the two planes are 300m apart, directly above eachother, calculate the distance from the observer to the runway.

Answer 1

Call the distance from observer to runway D and the height of the lower plane h. This yields two equations using tangent:

tan 18 = h/D
tan 25 = (h+300)/D

If you divide equation 1 by equation 2, you get:

tan18/tan25 = h/(h+300)
.6968(h+300) = h
.6968 + 209.04= h
h = 689.4215571m

Therefore, D = h/tan18 = 2121.8m

Answer 2

y + 300 = dtan25
y = dtan18
(y + 300)/y = tan25/tan18
y + 300 = ytan25/tan18
y(tan25/tan18 - 1) = 300
y = 300/(tan25/tan18 - 1)
d = 300/(tan18)(tan25/tan18 - 1))
d = 2,121.8 m