If the random variables $X$ and $Y$ have the joint density
$$ f(x,y) = \left\{ \begin{array}{ll} \frac{6}{7}x & \mbox{for $1 \leq x+y \leq 2,x \geq 0,y \geq 0$};\\ 0 & \mbox{otherwise}.\end{array} \right. $$
what is the density of $\frac{X}{Y}$?
I already know that the answer to this question is
$$ g_1(u) = \left\{ \begin{array}{ll} \frac{2u}{(1+u)^3} & \mbox{if $0 \leq u \leq \infty$};\\ 0 & \mbox{otherwise}.\end{array} \right. $$
How can we arrive to that answer? Any help will be appreciated.