He will be making a right triangle with the legs being 24 x 45. The diagonal will be the hypotenuse of the triangle. To find the amount of fencing we need the perimeter of the triangle.
Use Pythagorean theorem to find the length of the hypotenuse.
a^2 + b^2 = c^2
24^2 + 45^2 = c^2
576 + 2025 = c^2
2601 = c^2
sqrt 2601 = c
c = 51
Adding the three lengths the total fencing needed is
51+24+45=120 meters
