Answer
The magnitude of the force that one atom exerts on the other is $6\frac{C^6}{x^7}$. This force is attractive.
Work Step by Step
$U(x) = \frac{-C^6}{x^6}$
$F = -\frac{dU}{dx}$
$F = -6\frac{C^6}{x^7}$
The magnitude of the force that one atom exerts on the other is $6\frac{C^6}{x^7}$. The negative sign means that this force is attractive.
