Determine the slope and write the Cartesian equation of the line. Write the equation of the line by the origin of coordinates that has vector director of components (1,2)  determine the slope and write the Cartesian equation of the line.
My attempt:
Let $l$ a line such that pass for the origin. 
Let $a$ a director vector of $l$. we know  as $a$ is a director vector of $l$ then the components $(1,2)$ are in the line.
We have two points of the line.
We know the general equation for the line is $Ax+By+C=0$
Here i'm stuck can someone help me?

I take it you want the line through the origin and $(1,2)$.  The equation is $y=2x$.  This is in slope-intercept form (slope $2$, $y$-intercept $0$).  The slope is $m=\dfrac {\Delta y}{\Delta x}=\dfrac{2-0}{1-0}=2$.
