Slope of a line which passes through two point (x1,y1) and (x2,y2) is given by-
k=(y2-y1)/(x2-x1)

here k= 2
Here one point is Origin (0,0)
substituting the given values -
2 =(y-0)/(3-0)
y= 6

Similarly, for other point
2 =(4-0)/(x-0)
x =2


So x+y = 8