Awould be greatly appreciated!  

The tree was approximately 40.8ft tall.

You use the equation, a^2+b^2=c^2.
12^2 + 39^2 = x^2, add the results of 12^2 and 39^2 to make 1665 = x^2. You can take the positive root of both sides, 3√185, and it equals 40.80441152, which is rounded to 40.8.