[tex]x ^{3} + 5x {}^{2} - 8 r - 20 = 0 \\ x { }^{3} + 5x {}^{2} - 8r - 20(x {}^{3} + 5x {}^{2} ) = 0 - (x {}^{3} + 5x {}^{2} ) \\ - 8r - 20 = - (x {}^{3} + 5x {}^{2} ) \\ \\ - 8r - 20 + 20 = - (x {}^{3 } + 5x {}^{2} ) + 20 \\ \\ - 8r = x {}^{3} - 5x {}^{2} + 20 \\ \\ [/tex]
hope it is helpful
