Minimizing pipeline cost

A utility company must run a pipeline from a station on one side of a straight river to a plant on the other side. The river is $4$ miles wide, and the plant is located $20$ miles downstream from the point directly across the river from the station. The pipeline will run underwater in a straight line from the station to some point on the opposite bank, and then along the bank (over land) to the plant. Let $x$ be the distance (in miles), measured along the bank, from the point directly across from the station to the point where the pipeline comes ashore. Underwater pipe costs \6$million per mile, and land pipe costs$$4$ million per mile.