7/9 would be irrational, I think.
Answer:
Look below.
Step-by-step explanation:
Good ol' Pythagorean Theorem.
To find the short side of the larger triangle, we'll use this equation:

You can solve the rest on your own.
Use the same type of equation for the smaller triangle, except your b would be the hypotenuse, so that equation would look something like this:

Hope it helped! Also, there seems to be a help video, if you didn't understand this explanation, why don't you go ahead and watch it?
x^4 - 1
-------------
x^2 + 1
(x^2 +1) (x^2 - 1)
= ------------------------
x^2 + 1
= x^2 - 1 ..................this is your answer
If i am correct the answer should be 6.08