Answer
a. 12
b. 11 dB
Work Step by Step
a. Intensity is proportional to the square of the amplitude. The amplitude is 3.5 times greater, so the intensity increases by a factor of $3.5^2 =12.25 \approx12$.
b. The change in sound level depends logarithmically on the increase in intensity.
$$\Delta \beta=10log\frac{12.25}{1}\approx 11 dB$$