A force $F=4\widehat{i}-15\widehat{j}N,$ acts on a body resulting in a displacement of $6\widehat{i}.$ If the body had a kinetic energy of 7 J at the beginning of the displacement, the kinetic energy at the end of the displacement is

Given $F=4\widehat{i}-15\widehat{j}N$  
Displacement, s = $6\widehat{i}$  
Initial Kinetic energy, $K_i=7J$ 
Work done, W = F.s = $(4\widehat{i}-15\widehat{j}).6\widehat{i}=24J$ 
If $K_f$ be the final kinetic energy i.e at the end of the displacement, then according to work – energy theorem,
$W=K_f-K_i\Rightarrow 24=K_f-7\Rightarrow K_f=24+7=31J$