$$ \sqrt { 8x\cdot\sqrt [ 3\, ]{ 8x } } -\sqrt { x\cdot\sqrt [ 3\, ]{ x } } =\frac { 27 }{ 4 } \\\,\\4\cdot\sqrt { x\cdot\sqrt [ 3\, ]{ x } } -\sqrt { x\cdot\sqrt [ 3\, ]{ x } } =\frac { 27 }{ 4 } \\\,\\3\cdot\sqrt { x\cdot\sqrt [ 3\, ]{ x } } = \frac { 27 }{ 4 } \\\,\\\sqrt [ 3\, ]{ x^2 } = \frac { 9 }{ 4 } \\\,\\\left(\sqrt [ 3\, ]{ x }\right)^2 = \frac { 9 }{ 4 } \\\,\\\sqrt [ 3\, ]{ x } = \frac { 3 }{ 2 } \\\,\\x = \frac { 27 }{ 8 }.$$