# How do you solve y=x-4 and y=2x using substitution?

May 1, 2018

$x = - 4$
$y = - 8$

#### Explanation:

We know that $y = x - 4$. We also know that $y = 2 x$

This must mean that $x - 4 = 2 x$. By 'substituting' y = 2x in the first equation, we have got a new equation in a single variable.
By solving,
$- 4 = 2 x - x$(subtracting x from both sides)
$\implies x = - 4$

Now we know that $x = - 4$. $y = 2 x$ must mean that $y = 2 \left(- 4\right)$
$\implies y = - 8$

the value of $x$ is $- 4$ and $y$ is $- 8$

#### Explanation:

given equations are $y = x - 4 \mathmr{and} y = 2 x$
put $y = 2 x$in $y = x - 4$ then we have $2 x = x - 4 \Rightarrow 2 x - x = - 4 \Rightarrow x = - 4$ since $y = 2 x \Rightarrow y = - 8$

May 1, 2018

$x = - 4 \mathmr{and} y = - 8$

#### Explanation:

Substitution means plugging one equation to another to solve for a variable, so:

Let's plug $y = x - 4$ into $y = 2 x$

By plugging it in, you get $x - 4 = 2 x$

Move variables to one side and constants in another

$x - 2 x = 4$

$- x = 4$

$x = - 4$

Since we found x, we can find y by plugging x back into either equations. Answer would be the same. To prove this, I'll plug x into both equations.

$y = x - 4 = \left(- 4\right) - 4 = - 8$

$y = 2 x = 2 \left(- 4\right) = - 8$

So, $x = - 4 \mathmr{and} y = - 8$