The diagonal of the rectangle can be calculated by using the Pythagorean Theorem , because the diagonal and the two sides , length and width, of the rectangle , makes a right triangle .
Now we have the measure of length and width and we have to find the measure of diagonal
So according to the Pythagorean theorem

where , d is the diagonal , a is the length and b is the width
lets plug the values



Taking square root on both sides
d=70.49 Inches
d=70.5 inches
hence the length of one of the braces is 70.5 inches