The side x is the hypotenuse of the triangle.
And we know that the leg that is labeled with a line is also 6.
Use the Pythagorean theorem and solve for x.
a² + b² = x²
3² + 6² = x²
9 + 36 = x²
45 = x²
√45 = x
√9×5 = x
√9√5 = x
3√5 = x
The answer is 3√5
Hope this helps :)