equations

x = 5 - 2x

+ 2x to both sides

x + 2x = 5

3x = 5

x = 5/3

x + 2y = 8

plug in 5/3 for x

5/3 + 2y = 8

- 5/3 from both sides

2y = 8 - 5/3

2y = 6.333...

1/2 to both sides

2y / 2 = 6.333... /2

y = 3.1666...