\(\mathrm{f}_{\mathrm{a}}^{\prime}=\frac{\mathrm{V}}{\mathrm{V}+\mathrm{V}_{\mathrm{s}}} \mathrm{f}_{\mathrm{o}}\)
\(\frac{f_{a}^{\prime}}{f_{a}}=\frac{V-V_{s}}{V+V_{s}}=\frac{350-50}{350+50}=\frac{300}{400}=\frac{3}{4}\)
\(\mathrm{f}_{\mathrm{a}}^{\prime}=\frac{3}{4} \times 1000=750 \mathrm{Hz}\)