We can use the pathagorian theorum: A²+B²=C² now in this case it would be X²+Y²=2² we also know that X and Y are the same number in this problem because they have the same corresponding angle (triangle angles always add to 180 and since we know 2 of the angles are 45 and 90 the remaining angle will also be 45) Because of this we can change our equation to Z²+Z²=2² (I'm using Z so that we do not confuse it with another letter we've already used) from there we can go to Z∧4=2² and then we will solve the equation. Z=√2 Z also equals X and Y therefore Y=√2
I hope this helps and please let me know if there is anything you don't understand I would be happy to clarify!
Explanation: The mode is the most frequent value, so it is the value that shows up the most. In this case, that would be the value "12" since it shows up three times (while the other values only show up once).