Find the equation of the line passing through the pair of points.

( 6,  13), ( 15,  21)

We can first find the slope (m).......this is the change in y /  the change in x

Note that, from the first point to the second, y changes by a +8 and x chages by a +9....so....the slope is

+8 / + 9  = 8/9

Now, letting one of the points be (x1, y1) - either is ok, I'll use the first - we have

y - y1 = m (x - x1)    so

y - 13 = 8/9 (x - 6)     simplify

y - 13  = (8/9)x  - 48/9

y - 13  = (8/9)x  - 16/3     convert 13 to 39/3   and add to both sides

y = (8/9)x - 16/3 + 39/3

y =(8/9)x + 23/3

And there you go.......check that the second point "works" in this equation - put "15" in for x and see if you get back "21" for y.......