Who ever can give me the right answers first gets a brainleist answer i need

The first answer is The cost increases by 2.50$ every additional pair of shoes

There should be a equation that the sequence follows.
Answers:
1 - the cost increases by $2.50 for every pair of shoes.
Equation: S = 2.5n
Where n is the number of pairs. 

2 - we need to first graph the equations, then compare the two. The formula is typically written as; Y = mx + C
Rearrange the original
y - c = ?(x - 9)
y = a(x - 9) + c
y = ax - 9a + c
Substitute each points given into; y = ax - 9a + c
(x,y)
(9,7) = x=9 ; y=7
7 = a(9) - 9a + c 
c = 7 
a = 0.
b) x=4 ; y=-8
-8 = a(4) - 9a + 7
-8 = -5a + 7
-15 = -5a
a = 3.
ANSWER: A. (9,7)
Please look at the formula: y=mx +c.

Hope this helps. Lets hope I didnt make any mistakes.