A.
Let t be the number of t-shirts you buy and s the number of shorts you buy.
Since each t-shirt you buy cost $4.50 the total cost for t-shirts is:

Since each short you buy cost $6 the total cost for shorts is:

The total cost of your purchase is:

Since you have a total of $108 the equation that models the possible combinations of your purchase is:

B.
If you buy 6 shorts that means that s=6, plugging this value into the equation above and solving for t we have:

Therefore, If you purchased 6 shorts then you can buy 16 t-shirts.
C.
If you buy 10 shirts that means that t=10, plugging this value into the equation in part A and solving for s we have:

Now, since you can't buy half of a short the answer above means ( that:
If you buy 10 shirts then you can buy a maximum of 10 shorts, this wil be a total of $103