Answer:
x = 81
Steps:
It says that y ~ √x
We have points (9, 18) as a guide.
We can write y = f(x) = a√x +b
Use points (9, 18) as a guide
18 = a√9 +b
18 = 3a + b
One solution might be a = 6, which means f(x) = 6√x
Thus when y = 54,
54 = 6√x
9 = √x
81 = x