Question

What did she do wrong ?

Answer 1

She wrongly added the equations

Explanation:

Given

`-3x + y = 8`

`-3x + y = -4`

Required

Her mistake

The mistake is when she added the equations.

When both equations are added, the result is:

`-3x -3x + y+y=8-4`

`-6x +2y=4`

and not

`2y = 4`

The suggestion to avoid such mistake is for the student to check the appropriate signs of each term before adding/subtracting.