The give side measures are 6, 12, 18.

Since the measure of the longest side is 18, So let c = 18, a = 6 and b = 12.

Then apply Pythagorean theorem: .

           (Substitute c = 18, a = 6 and b = 12)

Evaluate powers: .

Since , the triangle is not right triangle.

The measures , the triangle is not right triangle.