For this case we have:
Cost of manufactures of T-shirts [tex]C(x)=10.5x[/tex]
where x represents the number of T-shirts
Part A:
Substituting [tex]x = 8[/tex] in the total cost equation you have to:
[tex]C (8) = (10.5) (8)\\\\C (8) = 84\\[/tex]
Thus, the cost of 8 shirts will be [tex]C (8) = 84\\[/tex]
Part B:
If [tex]x = 12[/tex] then
[tex]C (12) = (10.5) (12)\\\\C (12) = 126\\[/tex]
Thus, the cost of 12 shirts will be [tex]C (12) = 126\\[/tex]
Answer:
[tex]C (8) = 84\\\\C (12) = 126\\\\[/tex]