a$)$ A hamonic troveling wave is moving in the positive $z$ direction with an amplitude of $5cm,$ frequency of $5Fc,$ and velocity of $30c\frac{m}{s}.$ Its displacement at the origin is $3cm$ at time zero. Write a wave equation for this wave that exbibits directly both propagation constant and angular frequency

b$)$ A thin film of oil $)=(1\mathrm{.}4$ is located on smooth, wet pavement. When viewed perpendicular to the pavemuent, the film reflexts most strongly red light at $672nm$ and reffects no blue light at $448$ nom. How thick is the ont thin?