How many people must attend the third show so that the average attendance per show is 3000

An average is just the total attendance numbers / the number of performances.

Let’s let the number of people in the third show be equal to x so we can write this as a formula, it’s easier to read. x is just "some number we don’t know"

The average is

Average=Total attendance/3
So
3000=(2580+2920+x)/3

So let’s multiply by that 3 on both sides, make x easier to find

Average*3=2580+2920+x ( * means multiplied)

The average is 3000

So

2580+2920+x=3000*3
2580+2920+x=9000

Then we subtract those other attendances to get x on its own.

x=9000-2580-2920
x=3500