a)

$4^3\cdot 4^5=4^{3+5}=4^8$

b)

${\left(5^4\right)}^3=5^{4\cdot 3}=5^{12}$

c)

${\left(6^2\right)}^4=6^{2\cdot 4}=6^8$

d)

$\frac{7^6}{7^4}=7^{6-4}=7^2$