What error did she make?
She simplified the denominator incorrectly. The denominator simplifies to –7.
She labeled the points incorrectly. The point (–7, 4) should be (x1, y1).
She used an incorrect formula. The formula should be the change in y-values with respect to the change in the x-values.
She used an incorrect formula. The formula should be the sum of the x-values with respect to the sum of the y-values.