The word referring to the long side of a triangle is called the hypotenuse.
The formula for finding the length of one of the sides of a right angle triangle is a2+b2=c2, with a and b the legs and c the hypoteneus. If the triangle is Isosceles, which means that 2 of its sides are equal, the length of each leg would be aproximately 7.778184593
you have to do 7 squared + 13 squared = x squared. 7 squared is 49 + 13 squared which is 169. 169+49=x squared. 218= x squared. The square root of 218 is about 14.765. So the answer should be about 15cm if the missing side is the hypotnus (the diaganal side)