A15-foot ladder is leaning against a building, with the base of the ladder 3 feet from the building. how high up on the building will the top of the ladder reach?

Here we basically use Pythagoras' theorem
looking at the picture above, I have drawn it out to make it clearer.
Pythagoras' theorem is : a^2 + b^2 = c^2
in this case we are trying to find the height so we would do the hypoteneuse ( longest side )
minus the smaller side so:

15^2 - 3^2
= 225 - 9
=216
now finally we would square root 216
=14.7ft

hope I helped :)