We're assuming the TV is a rectangle, meaning all corners have right angles.
From the given information, we have a right triangle with a height of 39 inches, hypotenuse, of 80 inches, and an unknown base width.
Using Pythagorean's theorem:


c = 80 and a = 39


Using a calculator and rounding to the nearest tenth gives

Let me know if you need any clarifications, thanks!