Easier to have it in the form
y=mx+c
y =x/3 + 14/3
Unfortunately not whole numbers so it won't go perfect through the edges of the boxes
Then you just make some x and y values yourself
Good starting point is 0 for both
ie when x is 0 then y =14/3
when y = 0 then x = -14
Which is off the graph in this case.
But using that idea if you pick an x or y value yourself you should be able to calculate its corresponding value.
You technically only need 2 sets of coordinates as it is a straight line but if you calculate more you can guarantee your line is correct