We apply the pythagorean theorem to find this answer.
a = 11 and b = 6 are the given legs
c = unknown hypotenuse
So,
a^2+b^2 = c^2
c = sqrt( a^2+b^2 )
c = sqrt( 11^2 + 6^2 )
c = sqrt( 121 + 36 )
c = sqrt( 157 )
c = 12.52996 approximately
c = 12.5
Side note: once you replace 'a' and b with 11 and 6, you can compute everything with a calculator in one single step more or less. The steps above are shown if you wanted to find the exact value sqrt(157).
