Combine all like terms to simplify the expression.

Step-by-step explanation:
The two terms that are alike are 14x and x.
Add both of the terms;
[tex]15x+3x^3-9[/tex]
Format the trinomial, exponents first, then variable(s);
[tex]\boxed{The \: correct \: option \:is \:C. \:3x^3+15x-9}[/tex]