The answer to your question is Yes ! Many people know how to solve this.
It's actually pretty interesting, so I'll run through it for you:
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Each corner of the screen is a right angle, so any diagonal
divides the screen into two right triangles, of which the diagonal
is the hypotenuse.
The legs of any of those right triangles are 7.2-ft and 9.9 ft,
and Pythagoras tells us that the length of the hypotenuse is
√(7.2² + 9.9²)
= √(51.84 + 98.01)
= √149.85
= 12.2413... ft
Rounded to the nearest tenth of a foot, that's 12.2 ft.