An equation of the line that contains the rigin and the point (1,2) is y= 2x.
Going from (0,0) - (1,2) your x goes up by 1 and your y goes up by 2. Slope equals rise/run which equals the change in y and x equalling 2/1 with an answer of 2. In the slope-interceot form: y= 2x + 0 or just simply y= 2x which brings you back to your equation of the line of y= 2x.