I included an illustration of what they are asking for.
Notice, since the light of the sun is coming from the same place, the shadow angles relative to the ground, X and Z, will be the same. So the triangles formed are similar
Use SOH CAH TOA to find the angle Z
This just means when you have an angle in a right triangle, in order to know whether to use Cosine, Sine or Tangent, look at what sides you have
We have the sides opposite (O) and adjacent (A) to angle Z, so we tangent (T)