Answer
The force is zero when $~~r = 0.2~nm$
Work Step by Step
We can write an expression for $F$:
$F = -\frac{dU}{dx}$
When $r = 0.2~nm$, the slope of the $U$ versus $r$ curve is zero.
Then:
$F = -\frac{dU}{dx} = 0$
The force is zero when $~~r = 0.2~nm$
