#### Answer

$a.) \ 0$
$b.) \ \dfrac{15+14x}{50x^2}$

#### Work Step by Step

$a.) \ \dfrac{a}{4}-\dfrac{2a}{8} = \dfrac{a}{4}-\dfrac{2a}{2(4)} = \dfrac{a}{4}-\dfrac{a}{4} = 0$
$b.) \ \dfrac{3}{10x^2}+\dfrac{7}{25x} = \dfrac{75x+70x^2}{250x^3} = \dfrac{5x(15+14x)}{5x(50x^2)} = \dfrac{15+14x}{50x^2}$