The guy wire, the telephone phone and the ground form a right-angled triangle.
Your measurements are x, x+4, and 20.
To solve for x, you could use the Pythagorean Theorem, a^2+b^2=c^2.
First, plug in the values for a, b and c, and then simplify.
From here, solve for x by either factoring, or using the quadratic formula (I'm going to use the quadratic formula).
Now, plug in the values of x into the original formula
The measurement of the telephone pole is x.
My values of x are 12 and -16.
If we substitute the values of x in,
only x=12 makes sense,
because a telephone pole with height -16 is impossible
hope it helps.