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.
