0. \( \frac{3^{x+1}-4 \cdot 3^{x-1}}{3^{x-2}} \)

Let's simplify the expression step by step. We start with: \[ \frac{3^{x+1} - 4 \cdot 3^{x-1}}{3^{x-2}}. \] First, we can rewrite the terms in the numerator: \[ 3^{x+1} = 3^{x-2} \cdot 3^3 = 27 \cdot 3^{x-2}, \] and \[ 4 \cdot 3^{x-1} = 4 \cdot 3^{x-2} \cdot 3^1 = 12 \cdot 3^{x-2}. \] Now, substitute these back into the expression: \[ = \frac{27 \cdot 3^{x-2} - 12 \cdot 3^{x-2}}{3^{x-2}}. \] Factoring out \(3^{x-2}\) from the numerator gives us: \[ = \frac{(27 - 12) \cdot 3^{x-2}}{3^{x-2}}. \] Since \(3^{x-2}\) in the numerator and denominator cancel out (provided \(3^{x-2} \neq 0\), which it isn't for real numbers \(x\)), we have: \[ = 27 - 12 = 15. \] Thus, the simplified expression is \[ \boxed{15}. \]