I'll try to explain since I can't draw a picture
starting at the tree draw a segment 5 ft
starting at the tree draw a segment 12 ft high
both with the same vertex
now connect them and you will form a right triangle
apply the Pythagorean Theorem
5^2 + 12^2= d^2
25+144= d^2
169= d^2
13=d (square root of 169)
distance = 13 feet
hope you can follow the steps