Dichlorobenzene has three isomers. The two C-Cl bonds lie in the plane of the ring, at 60° in the…

Step 1. Ortho, $60^\circ$. $2 \times 1.55 \times \cos 30^\circ = 3.10 \times 0.866 = 2.68\ \text{D}$.

Step 2. Meta, $120^\circ$. $2 \times 1.55 \times \cos 60^\circ = 1.55\ \text{D}$.

Step 3. Para, $180^\circ$. $\cos 90^\circ = 0$, so the two dipoles cancel: $0\ \text{D}$.