First you need to find at least two dimensions of the triangle containing a side length of x. The triangle of the top can be used for that. The hypothenuse for the 7,3 triangle is .
Now we can use the other triangle with sides of x,, and a hypothenuse of 9 to find x.
x^2 + = 9^2
simplify
x^2 + 58 = 81
subtract 58 from both sides
x^2 = 23
x =
and since this radical cannot be simplified further that is your answer.