26362

Since square 1 is 2,2, and square 2 is 3,5, the pattern here is that the first coordinate is one higher than the square's number. The second coordinate grows by three every time, so each time it grows two higher than the other coordinate.
You can take away one from the square number and multiply it by two - the differnce between what square you're working on, and the first square. The 2 is the difference the two coordinates are seperated by each square, so the difference for the 20th square should be 38 (19 x 2), so you add it on to the first coordinate (21), to make 59. (21,59 is my answer.)