What is the coefficient of the second term of the trinomial?

 " 18 " .

Explanation:

  (3x + 3)²
 
 =   (3x + 3) (3x + 3)  ;
 
 =  Using "FOIL" method:  "First, Outer, Inner, and Last" terms ;

First terms:  3x* 3x = 9x² ;
Outer terms:  3x * 3 = 9x ;
Inner terms:  3 * 3x = 9x ;
Last terms:   3 * 3 = 9 ;

   So,  we have:  " 9x² + 9x + 9x + 9" ;

      Combine the "like forms" to simplify further:

          The "like terms" are:  + 9x + 9x = 18x ; 

 Rewrite the expression:  

          " 9x²  + 18x +  9 ";

We want to find the coefficient of the second term in the trinomial given:

          " 9x² + Bx + 9 "  ; 

The coefficient of the second term in this polynomial is: "B" ;

                B = 18 .

The answer is:  " 18 " .