Discussion
I'm going to presume that you mean 
and 
and that the figure looks something like:
Don't worry about the orientation of the figure (tall-thin vs. short-fat)
because that will all sort itself out when we solve the problem.
Conceptually, this is a relatively straight-forward Pythagorean Theorem problem.
The only difficulty will be the arithmetic because these numbers are truly
ugly. Either your instructor or the author of your book has a mean streak.
Also, you won't be able to determine a fixed value for ET because both of the
rectangle's sides are given in terms of x. The best you will be able to do
is present an expression for ET in terms of x.
Solution
The segment ET makes the hypotenuse of a right triangle and either the segments
RE and RT or CT and EC are the legs. But since this is a rectangle, RE = CT
and RT = EC. So to apply Pythagoras' Theorem, we can say that a = RE and
b = EC. And finally, we can say that ET = c
Square the 'a' term, apply FOIL to the b expression, and gather terms:
If you have more information so that x can be determined, you can calculate
the value of that ugliness.
(