a)  ${\left(5^6\right)}^7=5^{6\cdot 7}=5^{42}$  

Odp. n=42

b)  ${\left(3^4\right)}^3=3^{4\cdot 3}=3^{12}$  

Odp. n=3

c)  ${\left({10}^6\right)}^5={10}^{6\cdot 5}={10}^{30}$  

Odp. n=6