# How do you solve by substitution y – 3x = 7 and 21 + 9x = 3y?

Mar 9, 2018

$\left(x , y\right) = \left(x , 7 + 3 x\right)$

#### Explanation:

Equation 1: $y - 3 x = 7 \implies y = 7 + 3 x$

Substitution : Substitute 7 + 3x for y in the equation 2:

$21 + 9 x = 3 y$
$21 + 9 x = 3 \left(7 + 3 x\right)$
$21 + 9 x = 21 + 9 x$
It is true for all real numbers => x can be any real number

$y = 7 + 3 x$

Equation 1 and 2 are the same line, and the answers is $\left(x , 7 + 3 x\right)$.