Given that $5x^2 + 2y^2 − 6xy + 4x − 4y \equiv a (x − y + 2)^2 + b (cx + y)^2 + d,$ find the values of the constants $a$, $b$, $c$ and $d$.
Solve the simultaneous equations \begin{align*} 5x^2 + 2y^2 − 6xy + 4x − 4y &= 9 , \\ 6x^2 + 3y^2 − 8xy + 8x − 8y &= 14 . \end{align*}