Question

solve
15r^2=3r

Answer 1

15r^2 - 3r=0
r(15r-3)=0

r=0 or r=15/3

Answer 2

15r^2=3r
-3r=-3r

15r^2-3r=0
3r(5r-1)=0

set each part "=0" and solve.
3r=0 5r-1=0
r=0 and r=1/5