please help me solve this problem using Excel solver. Please show the formulas used in solver. Step by step I am stuck. Thanks in advance.

A national catalog and Internet retailer has three warehouses and three major distribution centers located around the country. Normally, items are shipped directly from the warehouses to the distribution centers; however, each of the distribution centers can also be used as an intermediate transshipment point. The transportation costs ($/unit) between warehouses and distribution centers, the supply at the warehouses (100 units), and the demand at the distribution centers (100 units) for a specific week are shown in the following table: Distribution Center Warehouse A B C Supply 1 $12 $11 $7 70 2 8 6 14 80 3 9 10 12 50 Demand 60 100 40 The transportation costs ($/unit) between the distribution centers are Distribution Center Distribution Center A B C A $ $8 $3 B 1 2 C 7 2 Determine the optimal shipments between warehouses and distribution centers to minimize total transportation costs.