好難ar!!!!唔識計!!!!

1) ( 3X -y) -( x+2y)
= 3x-y-x-2y
=2x - 3y

2) (3x+y-5) - (2x-7y)+(x-2y-3)
= 3x + y - 5 -2x +7y +x -2y -3
=2x +6y - 8

3) 3a x 4b x c
= 12abc

4) (-12)x7+(-19)
= - 84 -19
= -103

5) 8a x 2b - 9ab
= 16ab-9ab
= 7ab