$(x^2 - 4)y' - 2xy = -2x(x^2 - 4)^2$
$\dfrac{d}{dx}\left(\dfrac{y}{x^2 - 4}\right) = -2x$
$y = (-x^2 + C)(x^2 - 4)$
For $x = 3,; y = 15$
$15 = (-9 + C)(5) \Rightarrow C = 12$
$y = (-x^2 + 12)(x^2 - 4)$
For maxima, $y' = 0 \Rightarrow x = 2\sqrt{2}$
$y_{\text{max}} = (-(2\sqrt{2})^2 + 12)\big((2\sqrt{2})^2 - 4\big)$
$= ( -8 + 12)(8 - 4) = 4 \cdot 4 = 16$