\[\boxed{\mathbf{21.}}\]
\[\textbf{а)}\ 3 + 2 \cdot 6 = 3 + 12 = 15\]
\[8 + 2 \cdot 3 = 8 + 6 = 14\]
\[11 - 2 \cdot 4 = 11 - 8 = 3\]
\[\textbf{б)}\ 30 + 2 \cdot 2 = 30 + 4 = 34\]
\[24 + 2 \cdot 5 = 24 + 10 = 34\]
\[50 + 3 \cdot 3 = 50 + 9 = 59\]
\[42 - 5 \cdot 2 = 42 - 10 = 32\]
\[39 - 2 \cdot 4 = 39 - 8 = 31\]
\[28 - 4 \cdot 2 = 28 - 8 = 20\]