ea=k*qa/r^2=9* 10^9*4* 10^-8/0.2^2=9000n/m

The direction to the left

Field strength excited by point b at point d

eb=k*qb/r^2=9* 10^9*8* 10^-8/0.4^2=4500n/m

Right direction

Therefore, the combined field strength at point D is E = 9000-4500 = 4500 N/m..

The direction to the left

The idea is this. I don't know if there is any mistake. Do it again yourself.