Hello, to find the side lengths you would use the Pythagorean theorem. For number 1 you would do 9^2+X^2=15^2
So 9 times 9 is 81 and 15 times 15 is 225. The new equation would be 81+X^2=225. To find X you would take 225 and subtract it from 81 to get 144. To get X you have to take the square root of 144 which is 12 so for #1 X will be 12. So for #2 you would take 6^2+X^2=10^2. The new equation is 36+X^2=100. You would do 100-36=64. 64=X^2. The square root of 64 is 8 so X would be 8. I hope I helped.