y = 0 when sin (3p t - 2.4p ) = 0
i.e., when 3p t - 2.4p = 0
t = 2.4/3 = 0.8 s
Of course it is also 0 every T (T = 0.667 s, see part b) from there.
i.e., 0.8 - 0.667 = 0.133 s



Is y increasing or decreasing as t is increased by D t at these points?

answer