Question 368812
We employ a "round-robin" technique...that is, every term on the first factor multiplies every term in the second factor...then we add them up...so

{{{(9x+3)*(7x^2+2x+1)}}}=
{{{63x^3 + 18x^2 + 9x + 21x^2 + 6x + 3}}}=  
now collect like terms and get
{{{63x^3 + 39x^2 + 15x + 3}}}