(x+3y)^7 = C(7,0)x^7(3y)^0 + C(7,1)x^6(3y)^1 + C(7,2)x^5(3y)^2 + C(7,3)x^4(3y)^3 + C(7,4)x^3(3y)^4 + C(7,5)x^2(3y)^5 + C(7,6)x^1(3y)^6 + C(7,7)x^0(3y)^7
(x+3y)^7 = x^7 + 7x^63y + 21x^5(3y)^2 + 35x^4(3y)^3 + 35x^3(3y)^4 + 21x^2(3y)^5 + 7x(3y)^6 + (3y)^7
(x+3y)^7 = C(7,0)x^7(3y)^0 + C(7,1)x^6(3y)^1 + C(7,2)x^5(3y)^2 + C(7,3)x^4(3y)^3 + C(7,4)x^3(3y)^4 + C(7,5)x^2(3y)^5 + C(7,6)x^1(3y)^6 + C(7,7)x^0(3y)^7
(x+3y)^7 = x^7 + 7x^63y + 21x^5(3y)^2 + 35x^4(3y)^3 + 35x^3(3y)^4 + 21x^2(3y)^5 + 7x(3y)^6 + (3y)^7