This question is based on combinations
Out of 7 appetizers, 4 are to be selected, this becomes = 7C4
Out of 3 main courses, 2 are to be selected , this is = 3C2
Out of 5 desserts, 4 are to be selected, this is = 5C4
So, total ways become =
7C4 * 3C2 * 5C4
7!/4!(7-4)! * 3!/2!(3-2)! * 5!/4!(5-4)!
= 35*3*5 =525 ways
Hence, this can be done in 525 ways.