Question

What is the period and amplitude for `y = 1/2sin (x/3 - pi)`?

Answer 1

The amplitude is represented by the number that multiplies the `sin`, i.e., `1/2`; so your sine oscillates between `+1/2` and `-1/2`.
The period is given by the number that multiplies `x` in the argument of sine, i.e., `1/3`, as:
`period=(2pi)/(1/3)=6pi`