Since the triangle is a right triangle, you can use the pythagorean theorem.
Let x be the length of the unknown side.
92 + x2 = (9√2)2
81 + x2 = 81·2 = 162
x2 = 162 - 81
x2 = 81
Take the square root of both sides.
x = ±9
Since the length cannot be a negative number, x = 9 m