Question 896858
y=-3x+4 and y=-4x-8
first put them in order 
x+y=b

3x+y=4
4x+y=-8

3,1,4
4,1,-8
A solution to these equations is the intersection of the lines

3,1,4
4,1,-8

divide row 1 by 3
1,1/3,4/3
4,1,-8

add  down (-4) *row 1 to row 2
1,1/3,4/3
0,-1/3,-40/3

divide row 2 by -1/3
1,1/3,4/3
0,1,40

We now have the value for the last variable.
We will work our way up and get the other solutions.

add up  (-1/3) *row 2 to row 1
1,0,-12
0,1,40

final
1,0,-12
0,1,40

1,0,-12
0,1,40
"-12","40"
(-12,40)