Calculus Vs Straight lines

$4xdx - 2ydy -3(xdy+ydx) = 0$
$4xdx - 2ydy -3d(xy) = 0$
Integrating,

$2x^{2} - y^{2} - 3xy = c$
Since this represents the joint equation of lines through the origin, c = 0.
$(2x-y)(x-y) = 0$

Thus , the two individual lines are $y =x$ and $y = 2x$
The acute angle between lines with slopes $m_{1}$ & $m_{2}$ is given by,
$\theta = \arctan \left| \dfrac{m_{1}-m_{2}}{1+m_{1}m_{2}} \right|$

Substituting $m_{1} = 2 , m_{2} = 1$
$\theta = \arctan \left(\frac{1}{3} \right)$

$a + b = 1 + 3 = 4$