Need Solution for Thermodynamics

[Maqboole]

I have an other problem of the same nature now. I have understood the question well but i dont understand what formula should be used for the solution.

Answer to the question is 7.579 W/m2 C

19 W

Water at 50 C flows inside 25cm diameter tube such that hi 350Wm-2 k-1.The tube has a wall thickness 0.8 mm with thermal Conductivity of 16W m-1 K-1.The outside of the tube loss heat by free convection with hoof 7.6 Wm-2 k-1. Calculate heat transfer cofficient & heat loss per unit lenght to surrounding air 20C.