1.
$(a+3b)^6$
$=C_6^0.a^6+C_6^1.a^5.3b+C_6^2.a^4.(3b)^2+C_6^3.a^3(3b)^3+C_6^4.a^2.(3b)^4+C_6^5.a.(3b)^5+C_6^6.(3b)^6$
$=a^6+18a^5b+135a^4b^2+540a^3b^3+1215a^2b^4+1458ab^5+729b^6$
2.
$(1+x)^{19}$
$=\sum\limits_{k=0}^{19}.C_{19}^k.x^k$
$x^7\Rightarrow k=7$
Vậy hệ số là $C_{19}^7=50388$