You need to use Pythagoras’ Theorem. A² + B² = c² and c is the hypotenuse of the right angled triangle.
The height of the tower (5 feet) is A and the distance from the end of the cable and the base of the tower (12 feet) is B. The length of ONE cable is c. So:
A² + B² = c² 5² + 12² = c² 25 + 144 = c² 169 = c² 13 = c. This is the length of one cable.
3 x 13 = 39 therefore the total length of the cables is 39 feet.