a) 6 x +21-4 = 7- 20+15 x

    6x +17 = -13+15x

    6x +17 +13 - 15x =0

    -9x +30 = 0

       x = -30/-9

        x = 30/9

         x = 3*10/3*3

         x = 10/3

b) 6x2 -8x+ 5= 6x2 - 15x - 12x

    6x2 - 8x + 5 - 6x2+15x+12x =0

     12x2+19x+5 =0