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a) The total interest is the rate times the money. However, since the rate is for 12 months, the interest will be half for just one semester. For the first account, 5.25%*4000$=210$. The amount is halved, so the interest is 105$. The second account yields: 6%*2000$= 120$. Hence, the interest is 60$
b) We have that the difference is 105-60=45$ (the interest from the 1st account)
c) The second account is the best because it offers a higher rate. For example, if we put 6000$  (the total amount we have available) in both accounts for a semester, the first one will yield 315$ while the second one will yield 360$.

2) So, first we would have that we get 145$ in our account. Then, we pay 65.34$. The next transaction are earnings. We can calculate them: 35*9.76=341.6$. That is what we get. Finally, the last transaction has a 4% tax. So, we need to pay 4%*61.32$=2.45$ dollars more.
We sum up all the amounts, putting a minus sign before our costs.
145$-65.34$+341.6$-61.32$-2.45$=357.49$.
The transactions will be put down like:
+145.00$
-65.34$
+341.60$
-63.77$ (the last transaction is one, tax sales is not applied separately).