Find two grid intersections that the line goes through. If it doesn't go through, make sure it has the same relationship to each of those grid intersections. I choose (4, 4) and (0, 1).
Then, subtract the y-coordinates to find their difference (∆y). Also subtract the corresponding x-coordinates to find their difference (∆x). Form the ratio (∆y)/(∆x). Reduce the fraction, if necessary. That ratio is your slope.
.. (4 -1)/(4 -0) = 3/4
The slope of the line appears to be about 3/4.