Question

How do you solve `4x+5y=21` and `-3x+ y =27` using substitution?

Answer 1

`color(brown)("Only demonstrating a different approach")`
This uses substitution to find `y` if you plug the value of `x` into one of the equations. However I should think they were looking for the method the others used.

Explanation:

Given

`color(white)(.....)4x+5y=21`...............................(1)
`color(white)(.)-3x+y=27`.................................(2)

Multiply equation (2) by 5 giving:

`color(white)(.......)4x+5y=21`...............................(1)
`underline(color(white)(.)-15x+5y=135)" "...............(2_a)`
`color(white)(......)19x+color(white)(.)0=-114" "->` Subtract equation `(2_a)` from (1)

`=> x=-114/19 = -6`

Then solve as the others did