s0.09=180
S is the total amount of students multiplied by 0.09, which is 9% in decimal form (9/100). 180 is the number of band members.
Now, we just need to solve:
s0.09=180
------- -----
0.09 0.09
s=2000
2000 students are enrolled at Anderson high.
Check:
2000(0.09)
=180