We can use the Pythagorean Theorem to solve for the length of the cable.
Remember that: A2 + B2 = C2
The values of the height of the tower and the distance from the base of the tower to the transmitter box would be "A" and "B".
Substitute the values we know:
(50)2 + (39)2 = C2
4021 = C2
(To solve for C, take the square root of both sides.)
C = square root of (4021) = 63.4 feet