Hi Niya,
I don’t see the model as you’re unable to attach diagrams here.
you have to use the trigonometric ratios to find the length of the support beam. From the description, we can understand that the support beam should be the hypotenuse of the right triangle. To find hypotenuse, you should use either sin or cos relationship.
Since the beam makes an angle of 39 with the vertical, and length of bird house is given, you can use sin 39= opposite/ hypotenuse relationship and solve.
hope this helps!