Tại sao ra 7?

\(x:\dfrac{15}{11}=\dfrac{-3}{12}:8\\ x^2+\dfrac{9}{25}=\dfrac{2}{5}:\dfrac{5}{8}\)

 \(\left(\dfrac{2}{3}-\dfrac{1}{2}-\dfrac{1}{3}\right).\left(1-\dfrac{1}{4}-\dfrac{1}{7}\right)\)

\(\left(\dfrac{15}{21}:\dfrac{5}{7}\right):\left(\dfrac{6}{5}:2\right)\)

\(-\dfrac{10}{21}.\left[\dfrac{9}{15}+\left(\dfrac{3}{5}\right)^2\right]\)