c
\( \mathrm{f}_{\mathrm{c}}=\frac{\mathrm{v}}{4 \ell_1} \quad \mathrm{f}_{\mathrm{o}}=\frac{\mathrm{v}}{2 \ell_2} \)

\( \left|\mathrm{f}_{\mathrm{c}}-\mathrm{f}_0\right|=7 \)

\( \frac{\mathrm{v}}{4 \times 150}-\frac{\mathrm{v}}{2 \times 350}=7 \)

\( \frac{\mathrm{v}}{600 \mathrm{~cm}}-\frac{\mathrm{v}}{700 \mathrm{~cm}}=7 \)

\( \frac{\mathrm{v}}{6 \mathrm{~m}}-\frac{\mathrm{v}}{7 \mathrm{~m}}=7 \)

\( \mathrm{v}\left(\frac{1}{42}\right)=7 \)

\( \mathrm{v}=42 \times 7 \)

\( =294 \mathrm{~m} / \mathrm{s}\)