\(\left(\dfrac{3}{8}-x\right)^3.\dfrac{1}{16}=50\%\\ \Rightarrow\left(\dfrac{3}{8}-x\right)^3.\dfrac{1}{16}=\dfrac{1}{2}\\ \Rightarrow\left(\dfrac{3}{8}-x\right)^3=\dfrac{1}{2}:\dfrac{1}{16}\\ \Rightarrow\left(\dfrac{3}{8}-x\right)^3=8\\ \Rightarrow\left(\dfrac{3}{8}-x\right)^3=2^3\\ \Rightarrow\dfrac{3}{8}-x=2\\ \Rightarrow x=\dfrac{3}{8}-2\\ \Rightarrow x=-\dfrac{13}{8}\)