yu hav 2 strate lines:
1...y=-4x+7
2...y=3.5x-9
yu want area belo line 1 & abuv line2
find weer 2 lines meet...y=3.5x-9=7-4x...7.5x=16, so x=16/7.5, x=2.133333333333333...
and y=7.46666666-9...y=-1.5333333
the point (2.13333...,-1.53333)
If yu thank av y-axis as the left edj, yu hav a triangel with vertikal line along y-axis
& 2 sides that gotu the point (2.13333..,-1.533333)