Hey there!
Since you are given the two shorter legs of this triangle, 3 and 9, and you are also given that it is a right triangle, you can find x by using the Pythagorean Theorem:
a^2+b^2=c^2
a and b are the two shorter legs and c is the hypotenuse, or x.
Now, plug in your known values:
(3)^2+(9)^2=c^2
Now, simplify and solve for c, or your value of x:
(3)^2+(9)^2=c^2
9+81=c^2
c^2=90
c=

Finally, because the answer wants an exact and simplified answer, we must simplify

to a radical in simplest form by factoring all the square factors from the radicand. In this case, the square factors of

is

*

which would simplify this radical to 3

.
Therefore, your final answer would be x=3

Hope this helps and have a nice day!