you have to divide 30 by six to see how much one pair costs and it would give you 5. Then you multiply by 8 to see how much 8 pairs would cost and you would get 40.
The current population (in 2005) is 4000 because it is the constant. The slope tells us how many people are added to the population, so 70 per year. For ex. 2008 (2008- 2005 = 3=> 3 x 70 = 210 => 210 + 4000 = 4210).
