Q. $\displaystyle (2x + y)dy - (4x +2y -1)dy = 0$

After using $\displaystyle Z = 2x +y$, I got the answer,

$\displaystyle c = 2y -ln(2x + y)$

However, its very different from the answer at the back of the book. I have checked and rechecked but ended up with the same wrong answer.