Exponential Equation 13Fa5B
1. **State the problem:** Solve the equation $$\frac{3^{z+2}}{9^{z-2}} = 81$$ for $z$.
2. **Rewrite bases to the same base:** Note that $9 = 3^2$ and $81 = 3^4$. So rewrite the equation as:
$$\frac{3^{z+2}}{(3^2)^{z-2}} = 3^4$$
3. **Simplify the denominator exponent:** Using the power of a power rule $\left(a^{m}\right)^n = a^{mn}$:
$$(3^2)^{z-2} = 3^{2(z-2)} = 3^{2z - 4}$$
4. **Rewrite the fraction with the same base:**
$$\frac{3^{z+2}}{3^{2z - 4}} = 3^4$$
5. **Use the quotient rule for exponents:**
$$3^{(z+2) - (2z - 4)} = 3^4$$
6. **Simplify the exponent:**
$$(z + 2) - (2z - 4) = z + 2 - 2z + 4 = -z + 6$$
So the equation becomes:
$$3^{-z + 6} = 3^4$$
7. **Since the bases are equal, set the exponents equal:**
$$-z + 6 = 4$$
8. **Solve for $z$:**
$$-z = 4 - 6 = -2$$
$$z = 2$$
**Final answer:**
$$z = 2$$