\[\boxed{\mathbf{34.}}\]
\[\textbf{а)}\ 11 - 2 + 4 = 9 + 4 = 13\]
\[11 - (2 + 4) = 11 - 6 = 5\]
\[30 - (13 - 7) = 30 - 6 = 24\]
\[30 - 13 - 7 = 17 - 7 = 10\]
\[\textbf{б)}\ 50\ ч - 12\ ч - 8\ ч =\]
\[= 38\ ч - 8\ ч = 30\ ч\]
\[50\ ч - (12\ ч - 8\ ч) =\]
\[= 50\ ч - 4\ ч = 46\ ч\]