I'm assuming that the triangle is a right triangle wherein the Pythagorean theorem is applicable.
a² + b² = c²
c = √a²+b²
c = √3² + 6²
c = √9 + 36
c = √45 unsimplified radical form
c = √9 5
c = 3 √5 simplified radical form
c = 3 2.24
c = 6.71 value of c to the nearest hundredth