What is the value of x?

Statement 1: √(x�) = 9
If we square both sides, we get:
[ √(x�) ]² = 9²
x� = 81.

The resulting equation does NOT have a root symbol.
Thus, when we solve for x, we must account for both the positive root AND the negative root:
(x�)^(1/4) = (81)^(1/4)
x = ±3.
Since it's possible that x=3 or that x=-3, the value of x cannot be determined.
INSUFFICIENT.

westie wrote:I thought that if the square root is written on the GMAT question, it implies positive roots only.

Correct:
√ = means the POSITIVE ROOT ONLY.
The equation in Statement 1 above abides by this rule:
√(x�) = 9.
Notice that √ on the left side yields ONLY A POSITIVE ROOT on the right side: 9.

This does not mean, however, that x itself must be positive.
As shown above, x=3 and x=-3 are both valid solutions for √(x�) = 9.
If we plug x=3 into √(x�) = 9, we get:
√(3�) = √81 = 9.
If we plug x=-3 into √(x�) = 9, we get:
√(-3)� = √81 = 9.
Thus, √(x�) = 9 implies that x = ±3.