How can I get from step $3$ to step $4$ in the problem?
I've tried this:
$6x^2 - 9x + 8x -12 + 2\Delta x - 6x^2 -8x +9x + 12 + 3\Delta x$
It doesn't cancel out to $17\Delta x$, though.
I've also tried this:
$(3x+4)(2x-3) - (2x-3)(3x+4) = 0$, then the only things left are $2\Delta x$ and $3\Delta x$. That adds up to $5\Delta x$, which isn't the answer.
How can I solve this problem?