Answer
$729 r^6 + 1458 r^5 s + 1215 r^4 s^2 + 540 r^3 s^3 + 135 r^2 s^4 + 18 r s^5 + s^6$
Work Step by Step
$(x+y)^n=\binom{n}{0}x^ny^0+\binom{n}{1}x^{n-1}y^1+...+\binom{n}{a}x^{n-a}y^a+..\binom{n}{n}x^{0}y^n$
Here: $n=6$, $x=3r$, $y=s$
$(3r+s)^6=\binom{6}{0}(3r)^{6}s^0+\binom{6}{1}(3r)^{6-1}s^1+\binom{6}{2}(3r)^{6-2}s^2+\binom{6}{3}(3r)^{6-3}s^3+\binom{6}{4}(3r)^{6-4}s^4+\binom{6}{5}(3r)^{6-5}s^5+\binom{6}{6}(3r)^{6-6}s^6=$
$729 r^6 + 1458 r^5 s + 1215 r^4 s^2 + 540 r^3 s^3 + 135 r^2 s^4 + 18 r s^5 + s^6$
