$[12$ marks$]$

Evaluate the following by using an appropriate trigonometric substitution. Give exact answers.

a$.(6$ marks$)$

${\displaystyle \underset{0}{\overset{\frac{3}{4}}{}}}x\sqrt[\phantom{2}]{9-4x^2}dx$

b$.(6$ marks$)$

${\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}\frac{x^2}{(25x^2{)}^{\frac{5}{2}}}dx$