Question

I cant figure out the mistake on #2. Please help?

Answer 1

The problem she commited was not substituting properly
x-y=7
subtracting x from both sides
-y=7-x
multiplying the whole equation by -1
y=-7+x
the first equation is

`2{x}^(2) - 6x + 4 = y`
so substituting it should be

`2 {x}^(2) - 6x + 4 = - 7 + x`
subtracting x it should be

`2 {x}^(2) - 6x + 4 - x = - 7`
which clearly means she did not substitute correctly