y'=-3-2x
b1=2, b2=6,
q=b2/b1=3,
S5=b1(1-q^5)/(1-q)=2(1-3^5)/(1-3)=242
3a^2+3ab-3ab+b^2+2nb-2na=3a^2+b^2+2nb-2na