A T-shirt department store cost 7.50. the total cost in dollar of a T-shirt is givien by the function C (a) = 7.5a

Given cost function C (a) = 7.5a, where a is the number of T-shirts.

Let us complete table.

We need to plug a=1,2,3,4 in above function to get the costs to complete the table.

For a = 1

C (1) = 7.5 (1) = 7.50

For a = 2

C (2) = 7.5 (2) = 15.00

For a = 3

C (3) = 7.5 (3) = 22.50

For a = 4

C (4) = 7.5 (1) = 30.00

In order to find the common difference we need to find the diffrences of costs 15-7.50 = 7.50

22.50-15.00=7.50.

Therefore, common difference is 7.5.

a represents the number of T-shirts.

Domain of a is the numbers we can take for a.

So, we can take value for a as 0 or greater value.

Therefore, domain for a would be a is gerater than or equal to 0.

Domain : a≥0