For this case we have:
Cost of manufactures of T-shirts 
where x represents the number of T-shirts
Part A:
Substituting
in the total cost equation you have to:

Thus, the cost of 8 shirts will be 
Part B:
If
then

Thus, the cost of 12 shirts will be 
Answer:

Answer: 4x² + 5x + 3
<u>Step-by-step explanation:</u>

20/35 and 21/35. The common denominator is 35.