X+y=8
2x-y=16
3x+z=18

The answer is (x, y, z)=(8, 0, -6)
You have to substitute the values
x=8-y
2x-y=16
3x+z=18
Then you simplify
2(8-y)-y=16
3(8-y)+z=18
Next you substitute the value of y
y=0
-3y+z=-6
After that you solve the equation
-3x0+z=-6
Then you substitute the value of y
z=-6
Simply
x=8-0
So a possible solution will be 8.
(X, y, z)=(8, 0, -6)
Simply
8+0=8
2x8-0=16
3x8+(-6)=18
The ordered triple is the solution
8=8
16=16
16=18
(x, y, z) = (8, 0, -6)

On monday,mrs.wise bought 3 lbs of apples at $4per lb= 4*3=$12on tuesday ,20%off and she buy 5 lbs of apples = $4-20%=3.20*5lb=$16example: other wise when she buy 5lbs on monday she have to spend 4*5=20