Answer
15 N
Work Step by Step
$W=-18\,J \implies\vec{F}\cdot\vec{d}= -18\,J$
$\implies F_{x}d_{x}+F_{y}d_{y}+F_{z}d_{z}=-18\,J$
$\implies (4.0\,N\times3.0\,m)+(c\times-2.0\,m)+(0\times0)=-18\,J$
$\implies 12\,J+(c\times-2.0\,m)= -18\,J$
$\implies (-2.0\,m\times c)= -18\,J-12\,J$
Or $c=\frac{-30.\,J}{-2.0\,m}=15\,N$
