a, \(\dfrac{3}{4}+x=\dfrac{8}{13}\)

\(x=\dfrac{8}{13}-\dfrac{3}{4}\)

\(x=-\dfrac{7}{52}\)

b,\(\dfrac{11}{12}-\left(\dfrac{2}{5}+x\right)=\dfrac{2}{3}\)

\(\dfrac{2}{5}+x=\dfrac{11}{12}-\dfrac{2}{3}\)

\(\dfrac{2}{5}+x=\dfrac{1}{4}\)

\(x=\dfrac{1}{4}-\dfrac{2}{5}\)

\(x=-\dfrac{3}{20}\)

c, \(2x\left(x-\dfrac{1}{7}\right)=0\)

\(2x-\dfrac{1}{7}=0\)

\(x-\dfrac{1}{7}=0:2\)

\(x-\dfrac{1}{7}=0\)

\(x=0-\dfrac{1}{7}\)

\(x=\dfrac{1}{7}\)

d, \(\dfrac{3}{4}+\dfrac{1}{4}\div x=\dfrac{2}{5}\)

\(\left(\dfrac{3}{4}+\dfrac{1}{4}\right):x=\dfrac{2}{5}\)

\(1:x=\dfrac{2}{5}\)

\(x=1:\dfrac{2}{5}\)

\(x=\dfrac{5}{2}\)