Question

How do you solve by substitution `x + y = 4` and `y = 2x + 1`?

Answer 1

`{x+y=4`
`{y=2x+1`
Take the second and substitute into the first for `y`:
`x+(2x+1)=4`
`3x=3`
`x=1`
Back into equation 2:
`y=2+1=3`
Solution:
`x=1`
`y=3`