2x+y=70

2y+z=110

X+y+z=?

Let's try this: add together the first 2 equations. We get 2x + 2y + y + z = 180

Subtracting y from both sides and adding z to both sides,

2x + 2y + y + z = 180

Subtract y from both sides. Then 2x + 2y = 180 - y

Add 2z to both sides. Then 2x + 2y + 2z = 180 - y + 2z

Dividing all terms by 2: x + y + z = 90 - y/2 + z

Here we have an expression for x + y + z in terms of 90, y and z.