Ok first take out the GCF which is 3 in this case.
You just have to substitute -2 in for x.
3(-2)+1
-6+1
-5
S + a = 722
s = 72 + a
(72 + a) + a = 722
2a + 72 = 722
2a = 722 - 72
2a = 650
a = 650/2
a = 325 <== adults
s = 72 + a
s = 72 + 325
s = 397 <== students
You would do, 4,200 over 8,000 equals x over 500,000 and then
x*8000=4200*500000
8000 = 2100000000
divide
x= 262500
262500 people