The <em>guy-wire</em>, the highest point of the <em>cell phone tower</em>, and the the <em>point on the ground that is 25 ft from the base of the tower</em> form a right triangle, where:
the <em>length of the guy-wire</em> is the hypotenuse of the triangle
<em>the height of the cell phone tower </em>is one leg (75 ft)
the distance from the <em>point on the ground to the base of the tower</em> is the other leg (25 ft).
Since you know the lengths of both legs, you can use Pythagora's theorem to find the hypotenuse (the lenght of the guy-wire):