$a) \frac{45}{4} - (2\frac{5}{7} + 5.25) $
$= \frac{45}{4} - (\frac{19}{7} + \frac{21}{4} )$
$= \frac{45}{4} - \frac{223}{28}$
$= \frac{23}{7}$
b) $\frac{5}{9} / 2.4 - \frac{41}{9} / 2.4$
= $(\frac{5}{9} - \frac{41}{9}) / 2.4$ = (-4) / 2.4 = $\frac{-5}{3} $
c) $( -\frac{3}{4} + \frac{5}{13}) \times \frac{7}{2} - ( \frac{9}{4} - \frac{8}{13}) \times \frac{7}{2}$
= $( -\frac{3}{4} + \frac{5}{13} - \frac{9}{4} + \frac{8}{13}) \times \frac{7}{2} $
= (-2) $\times \frac{7}{2} $ = -7