From \(C\) to \(B\) the time interval of travelling is same.

So, \(v_{ av }=\frac{v_2+v_3}{2}=\frac{2+4}{2}=3 \,m / s\)

Now, first half is covered with \(6 \,ms ^{-1}\) and second half with \(3 \,ms ^{-1}\). So when distances are same.

\(v_{ av }=\frac{2 v_1 v_2}{v_1+v_2}=\frac{2 \times 6 \times 3}{6+3}=4 \,ms ^{-1}\)

\(v_{ av }=4 \,ms ^{-1}\)