Answer:
B
Step-by-step explanation:
[tex] {18}^{2} = {x}^{2} + {x}^{2} \\ \frac{18 \times 18}{2} = {x}^{2} \\ x = \sqrt{9 \times 18} \\ x = \sqrt{9 \times 9 \times 2} \\ x = 9 \sqrt{2} [/tex]
