You have to use the Pythagorean Theorem
a^2+b^2=c^2
You use this to find the hypotenuse
We know one side of the rectangle is 17 and the other is 12.75, so they will be a and b
The sum of the squared a and b is 451.5625
We square that if we want to find c
Hypotenuse (c) = 21.25