Answer
${\bf 8900}\;\rm N$
Note that this is a rough estimation from the graph which means your solution may differ from ours.
Work Step by Step
To estimate the strength of the strong force between two nucleons separated by 1.5 femtometers, we use the concept that force is the negative of the slope of the potential energy graph.
$$F=-{\rm Slope}_U=-\dfrac{\Delta U}{\Delta r}$$
From the mentioned diagram, at $ r = 1.5 \, \text{fm} $, the potential energy $ U \approx -39\, \text{MeV}$.
We can estimate that the slope of the graph at this point is as follows;
A tangent line drawn to the curve at this point goes from $U=0\;\rm MeV$ to $U=-39\;\rm MeV$, and from $r=1.5\;\rm fm$ to $r=2.2\;\rm fm$.
$$F =-\dfrac{ 0-(-39)}{(2.2-1.5)\times 10^{-15}}\times 10^6\times 1.6\times 10^{-19} $$
$$F=\bf -8914.29\;\rm N$$
Note that this is a rough estimation from the graph which means your solution may differ from ours.
Thus, the strength of the strong force at this separation is given by
$$F\approx \color{red}{\bf 8900}\;\rm N$$
