In amplitude modulation, the amplitude of each sideband is given by:
\[
{Amplitude of side bands} = {Modulation index} \times \frac{{Amplitude of carrier signal}}{2}
\]
Given that the modulation index \( m = 0.4 \) and the carrier signal amplitude \( A_c = 28 \, {V} \):
\[
{Amplitude of side bands} = 0.4 \times \frac{28}{2} = 0.4 \times 14 = 5.6 \, {V}
\]