7. A wave propagating along the x direction according to Equation (5.18), has a maximum displacement of 4.0 cm at x = 0 and t = 0. The wave speed is 5.0 cm/s, and the wavelength is 7.0cm. (a) What is the frequency? (b) What is the wave's amplitude at x = 10 cm and t = 13 s? \Psi(x, t)=A \sin (k x-\omega t+\phi)