$a){\left(3^2\right)}^3:3^5=3^{2\cdot 3}:3^5=3^6:3^5=3^{6-5}=3$

$b){\left(\frac{5^4}{5^2}\right)}^2={\left(5^4:5^2\right)}^2={\left(5^{4-2}\right)}^2={\left(5^2\right)}^2=5^{2\cdot 2}=5^4=5\cdot 5\cdot 5\cdot 5=25\cdot 25=625$