Để B \(\inℕ^∗\)thì \(\frac{2a+11}{a+3}\inℕ^∗\)
\(\Rightarrow2a+11⋮a+3\)
\(\Rightarrow2\left(a+3\right)+5⋮a+3\)
\(\Rightarrow5⋮a+3\)( Vì \(2\left(a+3\right)⋮a+3\) )
\(\Rightarrow a+3\inƯ\left(5\right)=\left\{1;2;3\right\}\)
\(\Rightarrow a\in\left\{-2;-1;0\right\}\)
Vậy: Nếu \(a\in\left\{-2;-1;0\right\}\)thì \(B\inℕ^∗\)