\[\boxed{\mathbf{9.}}\]
\[\textbf{а)}\ 11 - 5 + 4 = 6 + 4 = 10\]
\[11 - (5 + 4) = 11 - 9 = 2\]
\[\textbf{б)}\ 19 - 7 + 2 = 8 + 2 = 10\]
\[19 - (7 + 2) = 19 - 9 = 10\]
\[\textbf{в)}\ 6 + (20 - 11) = 6 + 9 = 15\]
\[(9 + 6) - 11 = 15 - 11 = 4\]