If I understand what you are asking, you can use the median as an average since it is not affected by the mean ( median is a measure of center). Since a mean is affected by outliers, it might lower the score if there are very low outliers. But note that if there are outliers way <em>greater </em>it can increase the average. You can use the Inter Quartile Range if shown on a box plot ( measure of variability).
Firstly the equation would be y=7.7x+59
(A)The slope is 7.7 (B) and shows that there is a 7.7 charge per week the person attends.
(A)The y intercept is 59 (B) and is the initial fee that the gym charges.
The midpoint of (x1,y1) and (x2,y2) is

just average them
(4,1) and (-2,4)
the midpoint is

(1,-1.5)
1. alternate interior angles, x= -10
2. corresponding angles, x= 30