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
