I need help on number 6)

I think the easiest way to answer all of these is to just go ahead and find the equation of the line first. 

I'm pretty sure that by the time you reach this problem, you've learned to find
the equation of a line that passes through two given points.  You have two points:  (2, 100) and (5, 205).  The equation of the line through them is [ y = 35x + 30 ].

6).  The initial membership fee ... the cost for walking in the door and signing up,
before any months, is $30.  (Also the y-intercept.)

7).  The cost per month is $35.  (The slope of the line.)

8).  We already did that, up above.  [  y = 35x + 30 ].

9).  'x' in the equation is the number of months.
There are 12 months in one year.

y = 35(12) + 30 = $450