Basically, let x be the number of students in the hall.

So the question says stadium has 4/3 more students than in the hall.
Hence
stadium is 4/3 x
Hall is x

For the girls in the stadium they make up 3/5 of the students in the stadium. Hence
3/5 of 4/3 x

For the boys in hall, they make up 5/7 of students in hall, x.
So boys in hall 5/7x
Hence girls is 2/7x ——> (x - 5/7x = 2/7x)
So girls in stadium is 216 more than girls in hall
So 3/5 of 4/3x more than 2/7x by 216.
Hence the equation in the answers.
So u find x
That’s the number of students in hall.
So u need to find students in stadium which is 4/3 x.