Apilot can see the runway she is about to land on directly ahead. when looking at each end of the runway the angles of depression are 45 degrees and 42 degrees. if the airplane is 1500 ft off the ground, how long is the runway?

We develop two different right triangles in which the opposite side to the given angle of depression is 1500 ft and the length of the runway is the y.
                (45°) -->       tan (45°) = 1500 ft / y₁  
                                                     y₁ = 1500 ft
                (42⁰) -->        tan (42⁰) = 1500 ft / y₂
                                                      y₂ = 1665.92 ft
Finding the difference between two y's, the answer would be 165.92 ft. 

Whose mistake? can you give more details to what the question is asking?