Answer is choice A) 20/21
The first step is to use the pythagorean theorem a^2+b^2 = c^2 to find the missing side to be 21. Once you know this, you find the ratio of the opposite and adajcent sides. The opposite side is the leg furthest from the angle x (the side labeled 20). The adjacent side is the leg closest to or touching the angle x (the side labeled 21 which was found earlier).
So, tan(x) = opposite/adjacent = 20/21