You can
put this solution on YOUR website! .
Let "a" be the angle which the incline makes with the horizontal line.
Then, after decomposition the weight force along and perpendicularly to the incline you get for the force along the incline
F = W*sin(a).
Then F is exactly the force needed to hold the piano on the inline.
In your case, W and F are given, and you obtain for the angle "a" sin(a) =
=
= 0.712.
It gives you a = arcsin(0.712) = 0.792 radians = 45.4 degrees.
Solved.