Susan and mark are standing at different places on a beach and watching a bird. the angles of elevation they make are 20º and 50º, respectively.
if susan and mark are 7 kilometers apart and the bird is between them, the bird is at a height of kilometers [blank] from the ground.

The picture is in the attachment.
x- horizontal distance between Susan and a bird
7 - x - horizontal distance between Math and a bird; 
h / x = tan 20° = 0.364
h / (7 - x) = tan 50°= 1.192

h = 0.364 x
0.364 x / (7 - x ) = 1.192
0.364 x = 1.192 ( 7 - x ) 
0.364 x = 8.344 - 1.192 x
1.556 x = 8.344
x = 8.344 : 1.556 = 5.362 km
h = 0.364 · 5.362 
h = 1.952 km 
.. the bird is at height of 1.952 km from the ground.