I wish I could draw a picture and add it here. I think it adds so much to this problem.
This is a special right triangle called an isosceles right triangle. Have you learned how to draw this?
In case you don't have this memorized, draw the right triangle. The legs are the same length.
Call each leg X. Then use the pythagorean theorem to find X
X2 + X2 = hypotenuse2
2X2 = (6√2)2
2X2 = 72
X = 6
both legs =6 . They are the height and length of the triangle
Area = (1/2)L·H = (1/2)(6)(6) = 18
Have a good night!