a,
\(x.\dfrac{1}{4}=\dfrac{1}{2}-\dfrac{4}{5}\)
\(x.\dfrac{1}{4}=\dfrac{5}{10}-\dfrac{8}{10}\)
\(x.\dfrac{1}{4}=-\dfrac{3}{10}\)
\(\Rightarrow x=-\dfrac{3}{10}:\dfrac{1}{4}\)
\(\Rightarrow x=-\dfrac{3}{10}.4\)
\(\Rightarrow x=-\dfrac{6}{5}\)
b,
\(\dfrac{2}{3}-x=\dfrac{1}{2}:\dfrac{3}{8}\)
\(\dfrac{2}{3}-x=\dfrac{1}{2}.\dfrac{8}{3}\)
\(\dfrac{2}{3}-x=\dfrac{4}{3}\)
\(x=\dfrac{2}{3}-\dfrac{4}{3}\)
\(x=-\dfrac{2}{3}\)