Picture a right triangle with a right angle formed by a line segment that is vertical and one that is horizontal. The two segments meet at the lower left of your triangle. The height of the triangle is the height of the tower. The wire of unknown length is the hypotenuse of the triangle. The angle given, 57 degrees, is the angle at the lower right of yout triangle (the angle between the ground and the wire).
You can find the missing sides of a right triangle either by using the Pythagorean Theorem or by some trigonometry. Since we only know one side, we can't use Pythagorean Theorem.
Trigonometry means we can relate sides of a triangle to one or more of the angles in the right triangle. Here, if we look at the 57 degree angle, we know the side opposite, 75 meters, and want to know the hypotenuse. Which trig function relates opposite/hypotenuse to an angle? That's what you need to solve this. Your answer will be larger than 75 meters, by the way.