\(a,x+\dfrac{4}{5}=\dfrac{45}{5}-\left(\dfrac{3}{7}+\dfrac{3}{5}\right)\)
\(x+\dfrac{4}{5}=9-\dfrac{3}{7}-\dfrac{3}{5}\)
\(x+\dfrac{4}{9}=\dfrac{279}{35}\)
\(x=\dfrac{279}{35}-\dfrac{4}{9}\)
\(x=\dfrac{251}{35}\)
\(b,\dfrac{5}{7}+\dfrac{x}{35}=\dfrac{4}{5}\)
\(\dfrac{5}{7}+\dfrac{x}{35}-\dfrac{4}{5}=0\)
\(\dfrac{25}{35}+\dfrac{x}{35}-\dfrac{28}{35}=0\)
\(25+x-28=0\)
\(x=3\)