Yes it is correct!
STEP BY STEP:
If Joe gave 5 pics to site A. Site A would give him $0.75*5=$3.75
If Joe gave 5 pics to Site B. Site B would give him $0.25*5=$1.25
3.75-1.25=2.50
Site A gave him $2.50 more than Site B.
64 is a perfect square that provides 8 rows , and each contains 8 seats, this is the first perfect square after 50
Assuming the variables are constants, you isolate m. So move am to the left. K-am=emx. Now divide ex. This gives you, m=(K-am/ex)