a)

$\frac{7^3:7^2\cdot 7^4}{7^5:7^2}=\frac{7^{3-2+4}}{7^{5-2}}=\frac{7^5}{7^3}=7^5:7^3=7^{5-3}=7^2=49$

b)

$\frac{{\left(-6\right)}^4:{\left(-6\right)}^3\cdot \left(-6\right)}{\left(-6\right)\cdot \left(-6\right):{\left(-6\right)}^2}=\frac{{\left(-6\right)}^{4-3+1}}{{\left(-6\right)}^{1+1-2}}=$