What is the length of large supporting wires from the top of a 45 feet tall building if they are stable to the ground 28 feet horizontally from the base of a tower?

you so much : )

53 feet.
You can use Pythagorean Theorem.

A right angled triangle is made, with the wire being the longest side (hypotenuse) and the building and ground being the two other sides at right angles. So,

a^2 + b^2 = c^2, c is the wire length.
so c = √(a^2 + b^2)

√(45^2 + 28^2) = √(2025+784)
= √2809 = 53 feet.