To find the length between the two opposite corners (or the length of the diagonal), we can use the Pythagorean theorem, where the two legs of the right triangle are the width and height.
Let x be the distance of the length.
The width must have a distance of 5x.
Now, we apply the Pythagorean theorem.

Simplify the terms items inside the square root

=

We can still simplify this further

That's your answer.
Have an awesome day! :)
~collinjun0827, Junior Moderator