How is profit calculated from trial balance and bank statements?

Put simply, profit = bank balance + income - expenditure.

Take a cake shop as an example. An accountant would note the opening balance of the shop's bank account at the start of the financial year. He would then add to that all the money taken in the shop over the year from selling cakes etc. Then he would deduct things like the cost of buying the cakes from the supplier, the running costs of the shop (electricity etc) and staff wages. Whatever figure is left after all the expenses have been deducted - is profit.