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.