12) (x⁹)⁸:(x⁵)⁷ (x⁴)⁹:x¹²

$$\frac{(x^9)^8 : (x^5)^7}{(x^4)^9 : x^{12}} = \frac{x^{9 \cdot 8} : x^{5 \cdot 7}}{x^{4 \cdot 9} : x^{12}} = \frac{x^{72} : x^{35}}{x^{36} : x^{12}} = \frac{x^{72-35}}{x^{36-12}} = \frac{x^{37}}{x^{24}} = x^{37-24} = x^{13}$$

Ответ: x¹³