3•(3a+7c)=

To simplify the expression \(3 \cdot (3a + 7c)\), you can distribute the 3 to both terms inside the parentheses:

\[ 3 \cdot (3a) + 3 \cdot (7c) = 9a + 21c \]

So, the simplified expression is:

\[ 9a + 21c \]