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
[tex] 2{x}^{2} - 6x + 4 = y[/tex]
so substituting it should be
[tex]2 {x}^{2} - 6x + 4 = - 7 + x[/tex]
subtracting x it should be
[tex]2 {x}^{2} - 6x + 4 - x = - 7[/tex]
which clearly means she did not substitute correctly
