Question

Solve each system by substitution
7.) x= -2y
x-y=9

Answer 1

`x + 2y = 0`

`x - y = 9`

Subtract the second equation from the first equation :

`(x + 2y) - (x - y) = 0 - 9`

`x + 2y - x + y = - 9`

`x - x + 2y + y = - 9`

Collect like terms

`0 + 3y = - 9`

`3y = - 9`

Divide both sides by 3

`(3y)/(3) = ( - 9)/(3) \\`

`y = - 3`

Now it's time to find the value of x so put the y value in one of the above equations to find the value of x :

`x = - 2 * y`

`x = - 2 * ( - 3)`

`x = 6`

Thus the solution is :

`s = (6 \: , - 3)`