the square is on the secant, not the (x - y)
you don't have many options here, what can we do with this. there are brackets on the right, multiply them out, we get:
we want to solve for , so get all the temrs with on one side of the equation
is a common term on the left, factor it out:
divide both sides by :
now this answer has a nice symmetry to it, but if you don't care about that, you can simplify the numerator on the right: