What is the amplitude of the sinusoidal function?

The amplitude is 4.

the amplitude is the distance from the maximum to the midline or the distance from the minimum to the midline. both are the same thing.

we can find the equation of the midline by taking the average of the maximum and minimum. the midline is in between the maximum and minimum.

the maximum is 1. the minimum is -7.

the average: add the two and divide by 2.

   

so the midline is at y=-3.

the distance between midline at y=-3 and the maximum at y=1 (take the absolute value because we do not care if a distance is negative or not) is

   

the amplitude is 4. you can count, from midline y=-3, count up to the maximum. that takes 4 steps.
then count, from midline y=-3, count down to the minimum. that also takes 4 steps.