The measure of central angle ABC is pi/2 radians.

What is the area of the shaded sector?

I know the answer is 9 pi units^2 but I just need an explanation

I cannot solve fully your problem because you did not provide the radius of the circle. but for the mean time, i will give you the formula for the area of a sector:

A = 0.5 (r^2) a

where r is the radius of the cirlce

and a is the central angle in radians.

since you provided the answer

9 pi = 0.5 (r^2) (pi/2)

r = 6