Determine the speed and direction of propagation of each of the following sinusoidal waves, assuming that x and y are measured in meters and t in seconds
(a) y = 0.60 cos(3.0x ─ 15t + 2)
(b) y = 0.40 cos(3.0x + 15t ─ 2)
(c) y = 1.2 sin(15t + 2.0x)
(d) y = 0.20 sin[12t ─ (x/2) + &]