I am guessing that you need an algebra equation so they are asking for an average of $1000 per day for 3 days which would be the $1000 times 3 and that has to equal the total earnings for Friday, Saturday, and Sunday. Friday and Saturday are known values and the unknown is (x) Sunday.
$1073 + 1108 + x = ($1000)*(3) ------ equation
$2181 + x = $3000 -------simplify
$2181 + x - $2181 = $3000 - $2181 ------ isolate x
x = $819 on Sunday to make the average of $3000 for 3 days