Need help with some Math

19)

21x  - 3y   = -42     divide through by  3  ⇒    7x  - y  = -14   (1)

y  = 7x + 14     (2)

Sub  (2)  into (1)     for "y"  and we have

7x -  [ 7x + 14]  = -14    simplify

-14   = -14       this is always true...whenever this happens....we have infinite solutions...!!!!

ok so I divide by 3 because there is a factor of 3 in each one?

20 )

x  - 2y   =  18      rearrange as    x  = 18  + 2y    (1)

-6x - 4y  = 4        (2)

Put  (1)  into (2)  for  x

-6(18 + 2y) - 4y   = 4       simplify

-108 - 12y  - 4y  = 4

-108 - 16y   = 4      add 108 to  both sides

-16y  = 112    divide both sides by  -16

y  = -7

And using (1)....   x  = 18  + 2(-7)   =  18  - 14    =  4