It's helpful here to draw a right-angle triangle, ABD.

A is the offshore well, D is the oil refinery.

AB = 2 km

BD = 4 km

C is somewhere between B and D such that BC = x and CD = 4-x

AC = path under the ocean = √(x²+2²)

CD= path on land = 4-x

Let y = cost per km on land

Cost of AC = 2y*√(x²+4)

Cost of CD = y(4-x)

Total Cost (C) = 2y√(x²+4) + y(4-x)

Hopefully you can take it from there.