The same question for different answers?

An oil refinery is located on the north bank of a straight river that is 2 km wide. A pipeline is to be constructed from the refinery to storage tanks located on the south bank of the river 6 km east of the refinery. The cost of laying pipe is $4 million/km over land from the refinery to a point P on the north bank, and $8 million/km under the river from point P directly to the tanks. To minimize the cost of the pipeline, where should P be located?