Using a graph tool
case 1)
2x-y=-13
y=x+9
the solution is the point (-4,5)
see the attached figure N 1
case 2)
y=3x-7
y=2x-5
the solution is the point (2,-1)
see the attached figure N 2
case 3)
3x+2y=10
6x-y=10
the solution is the point (2,2)
see the attached figure N 3
case 4)
y=6
x=-5
the solution is the point (-5,6)
see the attached figure N 4
case 5)
4x-3y=5
3x+2y=-9
the solution is the point (-1,-3)
case 6)
x+y=7
x-y=-1
the solution is the point (3,4)
the answer in the attached figure
Answer:
The answer is 17 or x is 17
$ 40 ..... 100 %
$ 28 ..... x %

The % of change is 70 %.
This is equivalent with:
- the final price ($ 28) is 70 % from the initial price ($ 40)
- the initial price ($ 40) was reduced with 30% giving the new price as: