Two point charges Q and $-5Q$ are placed at some distance apart. If the electric field at the location of Q is $\overrightarrow{E}$ , the field at the location of $-5Q$ is

Electric field at Q means, place 1C charge at A and force experienced is electric field.
Electric field due to -5Q at 

$A=\frac{1}{4\pi {\in }_0}\left(\frac{-5Q}{d^2}\right)=\overline{E}$ Now place 1C charge at B 

Now field due to Q at $B=\frac{1}{4\pi {\in }_0}\frac{Q}{d^2}=\frac{-\overline{E}}{5}$