If there is a hypotenuse, the triangle is a right triangle. From the Pythagorean Theorem:
c^2 = a^2 + b^2 [the square of the hypotenuse is the sum of the squares of the other two sides]
c^2 = 11^2 + 11^2 = 121 + 121 = 242
c = sqrt(242) = 15.56 cm (rounded)