Deducing sinusoidal expression for V
\$V(t) = \hat{V}Sin(\omega t + \theta) \$

So we know \$\hat{V}\$ = 200

We know T = 20ms -> f = 50Hz -> \$\omega = 100\pi \$

if V(t) = -75 when t=0 

\$V(0) = -75 = 200Sin(\theta) \$

\$\theta = Sin^{-1}(-75/200) = - 0.384\$

main expression = \$V(t) = 200Sin(100\pi t- 0.384) \$