is this right? Certainly. Replacing ???? k with your notation ???? f, the totalcost function becomes ???? ( ???? ) = ???? + ???? ???? 2 + ( ???? ???? ) 2 , C(D)=D+f N 2 +(MD) 2 , where ???? D = miles of line run above ground down 1st St., ???? M = vertical offset (miles north) of the station above Shopping Ln, ???? N = horizontal offset (miles east) of the center from 1st St., ???? f = cost multiplier for underground vs.\ aboveground (e.g.\ ???? = 3 f=3 means underground costs 3 as much per mile). You then differentiate ???? ( ???? ) C(D) with respect to ???? D, set ???? ( ???? ) = 0 C (D)=0, and solve for the optimal ???? D