ANSWER we need more information to answer because you told us the stuff to get the answer but what are we trying to answer?
To find the mean you add up all the numbers, then divide by how many numbers there is.
so for 6th grade you would add up 2+3+3+3+3+4+4+5+5+5+5+6+6+6+6+6+7+9+10+10=?
and then divide that by 20.
same with 8th grade.
and then you would subtract one from the other
the answer is the second option
If you have an equation, then the number before the x is your slope, if you have points, use the equation (y2-y1)/(x2-x1)