$$\eqalign{
& 4) \cr
& {\left( {x - 1} \right)^3} - {\left( {3 + x} \right)^3} = 2{\left( {2x + 1} \right)^2} - 3{\left( {1 - 3x} \right)^2} \cr
& \Leftrightarrow {x^3} - 3{x^2} + 3x + 1 - 27 - 27x - 9{x^2} - {x^3} \cr
& \,\,\,\,\,\,\,\,\,\,\, = 8{x^2} + 8x + 2 - 3 + 18x - 27{x^2} \cr
& \Leftrightarrow 7{x^2} + 7x - 25 = 0 \cr
& \Leftrightarrow x = {{ - 7 \pm \sqrt {749} } \over {14}} \cr
& 5)\,\,Chua\,ro\,\,de\,\,bai. \cr} $$