(a+6а)•(3+а)

To simplify the expression \((a + 6a) \cdot (3 + a)\), follow these steps:

1. Combine like terms inside the first parentheses: \[ a + 6a = 7a \] So the expression becomes: \[ (7a) \cdot (3 + a) \]

2. Now apply the distributive property: \[ 7a \cdot (3 + a) = 7a \cdot 3 + 7a \cdot a \]

3. Perform the multiplication: \[ 7a \cdot 3 = 21a \] \[ 7a \cdot a = 7a^2 \]

4. Combine the results: \[ 21a + 7a^2 \]

Finally, we can write it in standard form: \[ 7a^2 + 21a \]

So the simplified expression is: \[ 7a^2 + 21a \]