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

May 6, 2018

$x = - 1.25$ and $y = - 3.75$

#### Explanation:

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

$x + y = - 5$

$x + 3 x = - 5$

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

$x + 3 x = - 5$

$4 x = - 5$

$x = - \frac{5}{4}$

$x = - 1.25$

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

$y = 3 \left(- 1.25\right)$

$y = - 3.75$ or in fractions $y = - \frac{15}{4}$

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

$x + y = - 5$

$- 1.25 + \left(- 3.75\right) = - 5$

May 6, 2018

$x = - \frac{5}{4}$
$y = - \frac{15}{4}$

#### Explanation:

$y = 3 x$
$x + y = - 5$

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

So
$y = \textcolor{red}{3 x}$
$x + \textcolor{red}{y} = - 5$

$x + \textcolor{red}{3 x} = 5$
$4 x = - 5$
$x = - \frac{5}{4}$

$y = 3 x = 3 \cdot \left(- \frac{5}{4}\right) = - \frac{15}{4}$