-2≤x
所以x+2≥0
所以|x+2|=x+2
x≤3
所以x-3≤0
所以|x-3|=-(x-3)
|x+2|-3|x-4|
=(x+2)-3[-(x-3)]
=x+2+3(x-3)
=x+2+3x-9
=4x-7