The lines are 10° apart in latitude, so it is pretty easy to determine that about halfway between 70° S and 80° S is 75° S latitude.
The longitude is a little more difficult, as the lines are 30° apart. My guess is that the location of interest is about 1/3 of the distance between 30° W and 60° W, so is about 40° W. (1/3 of the 30° difference is about 10°, so we add that amount to 30° to get the longitude of interest.)
My read on the location is 40° W longitude, 75° S latitude.
