Nguyên Hạo,we need to find 2 fractions or 2 values of x,so the last row must be :

\(\Rightarrow-17,5< x< -15\Rightarrow x\in\left\{-17;-16\right\}\)(since \(x\in Z\))

So,2 satisfied fractions are \(\dfrac{-17}{21};\dfrac{-16}{21}\)

\(-\dfrac{5}{6}< \dfrac{x}{21}< -\dfrac{5}{7}\Leftrightarrow\dfrac{-35}{42}< \dfrac{2x}{42}< \dfrac{-30}{42}\)

\(\Rightarrow-35< 2x< -30\)

\(\Rightarrow-17.5< x< 15\Rightarrow x=16\)