Answer :

[tex](2x+1)(x+5)=2x^2+\boxed{? \ x}+5 \\\\ (2x+1)(x+5)= 2x^2+10x+x+5 \to 2x^2+\boxed{11x}+5 \\\\ ? \ x=11x \\\\ \boxed{?=11}[/tex]