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