Question

How do you solve by substitution `y=3x` and `x+y=-5`?

Answer 1

`x=-1.25` and `y=-3.75`

Explanation:

Using `y=3x` substitute this value of `y` into `x+y=-5`:

`x+y=-5`

`x+3x=-5`

Then simplify and rearrange to make `x` the subject of the equation:

`x+3x=-5`

`4x=-5`

`x=-5/4`

`x=-1.25`

Then substitute this value of `x` back into `y=3x` so you find the value of `y` (if it asks you for it):

`y=3(-1.25)`

`y= -3.75` or in fractions `y=-15/4`

To double check your answer, substitute values of `x` and `y` into `x+y=-5` to see whether you get the result of `-5`:

`x+y=-5`

`-1.25+(-3.75)=-5`

Answer 2

`x=-5/4`
`y=-15/4`

Explanation:

`y=3x`
`x+y=-5`

As we see `y` is equal to `3x` in the first equation.
`y` which is in the second equation must be replaced or substituted by `3x.`

So
`y=color(red)(3x)`
`x+color(red)y=-5`

`x+color(red)(3x)=5`
`4x=-5`
`x=-5/4`

`y=3x=3*(-5/4)=-15/4`