Answer:
288
Explanation:
From the first two parts of the problem, we have
18√8 - 8√18 = 36√2 - 24√2
 = (36 - 24)√2
 = 12√2.
Thus, we wish to solve for n in the equation 12√2 = √n. In other words, we want to know what number 12√2 is the square root of. Any nonnegative number is the square root of its own square, so 
n = (12√2)²
 = 12²(√2)²
 = 144 ∙ 2
 = 288.