X-y+z=-7
x+y+z=1
x+y-z=9

how would you solve this equation?

X - y + z = -7
x + y + z = 1
add
2x + 2z = -6

x + y + z = 1
x + y - z = 9multiply by -1

x + y + z = 1
-x - y + z = -9 (result of multiplying by -1)
add
2z = -8
z = -8/2
z = -4

2x + 2z = -6
2x + 2(-4) = -6
2x - 8 = -6
2x = -6 + 8
2x = 2
x = 1

x - y + z = -7
1 - y - 4 = -7
-y -3 = -7
-y = -7 + 3
-y = -4
y = 4

so x = 1, y = 4, and z = -4

69 i think because is the only one square

The discount is 8.70 . you dont tell us the percent of tax added so there is no way to find out, unless i'm not seeing anything. without tax its 37.70