Answer
-2.5 N
Work Step by Step
$W=17\,J \implies\vec{F}\cdot\vec{d}= 17\,J$
$\implies F_{x}d_{x}+F_{y}d_{y}+F_{z}d_{z}=17\,J$
$\implies (4.0\,N\times3.0\,m)+(c\times-2.0\,m)+(0\times0)=17\,J$
$\implies 12\,J+(c\times-2.0\,m)= 17\,J$
$\implies (-2.0\,m\times c)= 17\,J-12\,J$
Or $c=\frac{5.0\,J}{-2.0\,m}=-2.5\,N$