\(\dfrac{15}{x+2}=\dfrac{3^3+3}{10}\\ \Leftrightarrow\dfrac{15}{x+2}=\dfrac{27+3}{10}\\ \Leftrightarrow\dfrac{15}{x+2}=\dfrac{30}{10}\\ \Leftrightarrow\dfrac{15}{x+2}=3\\ \Leftrightarrow x+2=\dfrac{15}{3}\\ \Leftrightarrow x+2=5\\ \Leftrightarrow x=5-2\\ \Leftrightarrow x=3\)

\(\dfrac{4}{5}-\left(-\dfrac{2}{7}\right)-\dfrac{7}{10}\)

\(=\dfrac{4}{5}+\dfrac{2}{7}-\dfrac{7}{10}\)

\(=\dfrac{8-7}{10}+\dfrac{2}{7}\)

\(=\dfrac{1}{10}+\dfrac{2}{7}\)

\(=\dfrac{7+20}{70}\)

\(=\dfrac{27}{70}\)