Which is the amplitude of the graph of the sinusoid given by y=3sin(5x)?

Peak amplitude is 3.
Because the sinus function is cyclic sin(x) and sin(5x) can be treated the same (the 5 only shifts the x position of the peak, not it's magnitude).
sin(x) is always between -1 and 1, so the maximum is 1. The maximum is the peak, and because it is multiplied by 3, the peak amplitude is 3.