Using the line of best fit, estimate how may miles jack ran the 10th week; show work.
during which week would you expect jack to run a full marathon, 26.3 miles? do you think that you should round up or down to the nearest week? why?

Take two points and find the slope. 
(4,9) (6,13)
y2-y1/x2-x1
4/2=2=m
y=2x+b
9=8+1
b+1
y=2x+1
Set x to 10 to find the 10th week
y=20+1
21 miles
Set y=26.3 to find which week.
26.3=2x+1
2x=25.3
x=12.6 weeks or round to 13 weeks
I think he should round up because in more slopes in between points, there is a lesser slope than 2.
Hope this helps.

The first option would be correct if it were one half and not one