If we add 8x to both sides we have
12x - 3x^2 < x^2
If we subtract x^2 from both sides we have
12x -2x^2 < 0
If we divide both sides by 2 we have
6x - x^2 < 0
If we divide both sides by x we have
6 - x < 0
Thus we have 6< x
Say x = 7.
28 - 3 * 49 < 49 - 56
-119 < -7 which is true.
If you don't understand naything feel free to ask.