Answer
The new amplitude would be $B\sqrt{2}$.
Work Step by Step
Equation 32.29 can be used to give the intensity in terms of the magnetic field amplitude.
$$I=\frac{E_{max}B_{max}}{2\mu_o}$$
We use the fact that $ E_{max}= cB_{max}$
$$I=\frac{cB_{max}^2}{2\mu_o}$$
If the intensity is doubled, the magnetic field amplitude is multiplied by $\sqrt{2}$.
