I'll assume the sine wave is 0 at t=0, i.e. zero phase shift.
\(A \sin(2\pi (700)(25\times 10^{-6}))=53.3V\\ A \sin\left(\dfrac{7\pi}{200}\right) = 53.3V\\ A = \dfrac{53.3}{\sin\left(\dfrac{7\pi}{200}\right)} =485.719V\)
\(A \sin(2\pi (700\times (50\times 10^{-6})) = 105.956V\)