4)
I:3x+4y=67
II:5x-y=35
to eliminate y when they are added one of the equations must contain -ay and the other +ay (with a being some number)
in this case we have in I already a positive term and in II a negative, but they have different numbers
by using multiplication (property of equations) we can multiply II by 4 so the negative and positive parts match
II*4: 20x-4y=140
when I and 4*II get added this results in
3x+20x+4y-4y=67+140
23x=207
5)
in this case I and II were added which can be seen in the left side of the equation((3x+4y)+(5x-y)=8x+3y), so to get the right side we have to also add the right sides 67+35=102
so the full equation would be:
8x+3y=102
