hi torrential,

The washer radii are 4 and

When y=0,

Therefore the inner washer radii are 4.

The curve intersects the line when

The surface area of inner disc is subtracted from the surface area of outer disc, to find the surface area of a washer.

We use x as the radius since x=0 is the axis of revolution.

Integrating these surfaces from y=-3 to 3 finds the vor.

The surface of a disc is