How to use Master Theorem with strange format of $b$ parameter?
Not every recurrence falls within the bounds on the master theorem. Your recurrence is an example. However, by unrolling your recurrence, we can come up with an explicit formula:
$$
T(n) = 6(n+1) + T(n-1) = 6(n+1) + 6n + T(n-2) = \cdots = \\
6(n+1) + 6n + \cdots + 6\cdot 2 + T(0) = \\
6(n+1) + 6n + \cdots + 6\cdot 2 + 6\cdot 1 = \\
6 \sum_{m=1}^{n+1} m = 6\frac{(n+2)(n+1)}{2} = 3(n+2)(n+1).
$$