When will I stop taking problems from creators?

I don't create this problem. All of the credits go to Nguyễn Tất Thu, Đoàn Quốc Việt and Vũ Công Minh for writing this problem, and the book in general.

The original problem is:

Solve the following system of equations:

$\large \left\{ \begin{aligned} \sqrt{y^2 - 8x + 9} - \sqrt[3]{xy - 6x + 12} &= 1\\ \sqrt{2(x - y)^2 + 10x - 6y + 12} - \sqrt y &= \sqrt{x + 2} \end{aligned} \right.$

Calculate the value of $x^2 - y^2 + 2xy - 6x + 8y - 15$ with $x$ and $y$ are real numbers.