This requires the pythagorean theorem :
a^2 + b^2 = c^2...where a and b are the legs and c is the hypotenuse..and by the way, this can only be used on right triangles
a^2 + b^2 = c^2
6^2 + 7^2 = c^2
36 + 49 = c^2
85 = c^2...take the sqrt of both sides, eliminating the ^2
sqrt 85 = c
9.22 = c....so the hypotenuse is approximately 9.22 units (thats rounded)