a) $x\;-\;\frac{1}{4} = \frac{-3}{8}\times\frac{2}{5}$.

$\Leftrightarrow x\;-\;\frac{1}{4} = \frac{-6}{40}$

$\Leftrightarrow x\;-\;\frac{1}{4} = \frac{-3}{20}$

$\Leftrightarrow x = \frac{-3}{20}\;+\;\frac{1}{4}$

$\Leftrightarrow x = \frac{-3}{20}\;+\;\frac{5}{20}$

$\Leftrightarrow x = \frac{2}{20}$

$\Leftrightarrow x = \frac{1}{10}$

b) $\frac{x}{126} = \frac{-7}{24}\times\frac{-6}{11}$

$\Leftrightarrow  \frac{x}{126} = \frac{42}{264}$

$\Leftrightarrow  \frac{x}{126} = \frac{7}{44}$

$\Leftrightarrow  x = 126\times \frac{7}{44}$

$\Leftrightarrow  x = \frac{126\times 7}{44}$

$\Leftrightarrow  x = \frac{441}{22}$