помогите решить задачу! Очень важно!!!!!!!!!!!

Consider the following tables:
TABLE `invoice`
Fields:
id: BIGINT(11) PRIMARY_KEY, AUTO INCREMENT,
first_name: VARCHAR(255),
last_name : VARCHAR(255),
issue_date: DATETIME,
product_amount: DECIMAL(7,2),
tax_amount : DECIMAL(7,2),
total_amount : DECIMAL(7,2),

TABLE `invoice_items`
Fields:
id: BIGINT(11) PRIMARY_KEY, AUTO_INCREMENT,
id_invoice: BIGINT(11) , FOREIGN_KEY ON invoice.id
item_name: VARCHAR(255),
product_cost: DECIMAL(7,2),
tax_cost: DECIMAL(7,2),
total_cost: DECIMAL(7,2)

The invoice table stores invoice information and invoice_items stores the items of the invoice.
Invoice.product_amount contains the sum of all product_costs for each item belonging to the invoice.
Invoice.tax_amount contains the sum of all tax_costs for each item belonging to the invoice.
Invoice.total_amount contains the sum of all total_costs for each item belonging to the invoice.

Also keep in mind :
Total_cost = Product_cost + Tax_cost
Tax_cost = 15% * Product_cost

Please write a model which exposes methods for the following:
1. Create a new invoice
2. Add an item in an invoice
3. Remove an item in an invoice
4. Remove an invoice
5. Edit an invoice issue date
6. Edit the product cost of an invoice.