Sampling bandpass signal with spectral replications
For (a) you just need to know that sampling will create replicas of the original spectrum shifted by multiples of the sampling frequency. But don't forget the negative frequencies. Since $s(t)$ is real-valued, you have $|S(f)|=|S(-f)|$, and you also have to shift the contribution at negative frequencies (which is a mirror image of the part of the spectrum shown in the figure). Considering the frequency range [-70MHz,70MHz], replicas of the positive part of the spectrum (as shown in the figure) occur at $70MHz-k\cdot56MHz$, $k=1,2$:

$$f_1=14MHz,\quad f_2=-42MHz$$

Replicas of the negative part of the original spectrum in the range [-70MHz,70MHz] occur at $-70MHz+kf_s$, $k=1,2$:

$$f_3=-14MHz,\quad f_4=42MHz$$

So the answer to (b) is $f_1=14MHz=70Mhz-f_s$, with the last equality being the answer to (c).

Check this link for more information on bandpass sampling.