when you're finding slope with two given point you choose the second point of Y subtract the first point of Y. Second point of X subtract first point of X
slope is the change of Y over the Change of X
(3,9) is the second ordered pair so that will be you x2 and y2 because the point is after (0,3)
equation for slope: y2-y1/x2-x1
what you do is
9-3 = 6
3-0=3
6/3 if you simplify it it is 2.
the slope is 2
plug your numbers into the equation and isolate b which is the y intercept