Two waves are simultaneously passing through a string and their equations are :

$\mathrm{y}_{1}=\mathrm{A}_{1} \sin \mathrm{k}(\mathrm{x}-\mathrm{vt})$

$\mathrm{y}_{1}=12 \sin 6.28(\mathrm{x}-\mathrm{vt})$

$\mathrm{y}_{2}=5 \sin 6.28(\mathrm{x}-\mathrm{vt}+3.5)$

$\Delta \phi=\frac{2 \pi}{\lambda}(\Delta x)$

$=\mathrm{K}(\Delta \mathrm{x})$

$=6.28 \times 3.5=\frac{7}{2} \times 2 \pi=7 \pi$

$\mathrm{A}_{\text {net }}=\sqrt{\mathrm{A}_{1}^{2}+\mathrm{A}_{2}^{2}+2 \mathrm{~A}_{1} \mathrm{~A}_{2} \cos \phi}$

$\mathrm{A}_{\text {net }}=\sqrt{(12)^{2}+(5)^{2}+2(12)(5) \cos (7 \pi)}$

$=\sqrt{144+25-120}$

Ans. 7