s=2r+3x
Move 2r to the other side. Sign changes from +2r to -2r.
s-2r=2r-2r+3x
s-2r=3x
Divide by 3 for both sides
s/3-2r/3=3/3x
x=s/3-2r/3
Answer: x=s/3-2r/3
