Two waves are simultaneously passing through a string and their...

$\Deltaϕ = {kx}_{0} = \dfrac{2\pi}{\lambda}{x}_{0} = 2\pi \times 3 . 5\left(\lambda = \dfrac{2\pi}{k} = 1\right)$

$= 2\pi \times \dfrac{35}{10} = 7\pi$

${A}_{R} = \sqrt{{12}^{2} + {5}^{2} + 2 \times 12 \times 5 \times \cos ( 7\pi )} = \sqrt{( 12 - 5{)}^{2}} = 7mm$