12 (74)5.77 (76)4 =

12. $$\frac{(7^4)^5 \cdot 7^7}{(7^6)^4} = \frac{7^{4\cdot5} \cdot 7^7}{7^{6\cdot4}} = \frac{7^{20} \cdot 7^7}{7^{24}} = \frac{7^{20+7}}{7^{24}} = \frac{7^{27}}{7^{24}} = 7^{27-24} = 7^3 = 343$$

Ответ: 343