The Pythagorean theorem is a^2+b^2=c^2 where a and b are legs and c is the hypotenuse(longest side). you haven’t put a picture so I don’t know where 35 is a leg or the hypotenuse, but all you have to do is square the 2 shorter sides and add them and set it equal to the square of the longer side. You can do it!
The answer is D. Because the sq. rt. of 1 is 1, the sq. rt. of 9 is 3, and the sq. rt. of 25 is 5. When you take the sq. rt. of 50 you get 7.071067.
12^2 + 6^2 = x^2
144 + 36 = x^2
180 = x^2
13.416407865 = x
rounded
x = 13.4