This is another right triangle question. You have the 22 foot cable as your c value and the ground length of 15 feet as your "a" value you are looking for the length of the pole or your b value
Plug in the values you know into the pythagorean theorm
a^2 + b^2 = c^2
15^2 + b^2 = 22^2
225 + b^2 = 484
Solve the equation for b by subtracting 225 from each side and then taking the square root of both sides
b^2 = 259
b = square root of 259