x²-3x≥4
x²-3x-4≥0
(x-4)(x+1)≥0
x≥4或x≤-1
x²-3x≤ 18
x²-3x-18≤0
(x-6)(x+3)≤0
-3≤x≤6
则-3≤x≤-1或4≤x≤6
-3,-2,-1,4,5,6