below is the sheet that has been provided. The Problem a is solved, but the Problem b is still incorrect. Plz help me with the Problem b! Much appreciated!

Ewing Oil has oil fields in San Diego and Los Angeles. The San Diego field can produce up to 500,000 barrels per day, and the Los Angeles field can produce up to 600,000 barrels per day. Oil is sent from the fields to a refinery, either in Dallas or in Houston. (Assume that each refinery has unlimited capacity.) To refine 1,000 barrels costs $6,300 at Dallas and $5,700 at Houston. Refined oil is shipped to customers in Chicago and New York. Chicago customers require 400,000 barrels per day, and New York customers require 600,000 barrels per day. The costs of shipping 1,000 barrels of oil (refined or unrefined) between cities are shown in the file P05_74.xlsx. a. Determine how to minimize the total cost of meeting all demands. If your answer is zero, enter " 0 ". Flows (in 1,000s of barrels) Total Cost: \$ Ewing Oil data Unit shipping costs (per 100,000 barrels) To \begin{tabular}{|l|l|r|r|r|r|} \hline & & Dallas & Houston & New York & Chicago \\ \hline From & Los Angeles & $3,250 & $3,300 & & \\ \hline & San Diego & $3,400 & $3,350 & & \\ \hline & Dallas & & & $3,600 & $3,100 \\ \hline & Houston & & & $3,550 & $3,200 \\ \hline \end{tabular}