Two lines are parallel if they have the same slope.

For the lines 2x-y+2=0 and x+y-4=0, the slope is :

2x - y + 2 = 0

=> y = 2x + 2

slope = 2

x + y - 4 = 0

=> y = -x - 4

slope = 1

**The two lines have different slopes, so they are not parallel.**

2x-y+2=0 and x+y-4=0

Recall that parallel lines have the same slope. Rewrite the two equations in point slope form:

y = mx + b where m is the slope.

2x-y+2=0

(1) y = 2x + 2

x + y - 4 = 0

(2) y = x + 4

Equation (1) has a slope of 2 and equation (2) has a slope of 1. Since the slopes are not equal, they are not parallel.

We'll solve the system formed of the given equations of the lines and we'll check if it has solutions. If the system has no solutions, that means that the lines are not intercepting each other, so they are parallel.

We'll solve the system using substitution method. We'll change the 2nd equation into:

x+y = 4

x = 4 - y (3)

We'll substitute (3) in (1):

2(4 - y) - y = -2

We'll remove the brackets:

8 - 2y - y = -2

We'll combine like terms and we'll subtract 8 both sides:

-3y = -2 - 8

-3y = -10

We'll divide by -3:

**y = 10/3**

We'll substitute y in (3):

x = 4 - 10/3

x = (12-10)/3

**x = 2/3**

**The solutions of the system is: {2/3 ; 10/3}.**

**Since the system has a solution, that means that the lines are not parallel.**