Seven 1.0-cm-diameter tubes carrying steam at 100C are buried in a semi-infinite medium having a thermal conductivity

Seven 1.0-cm-diameter tubes carrying steam at 100ºC are buried in a semi-infinite medium having a thermal conductivity of 1.2 W/m· ºC and surface temperature of 25ºC. The depth to the centerline of the tubes is 5 cm and the spacing between centers is 3 cm. Calculate the heat lost per unit length for each tube.