A hollow tube possesses a length l , inner radius R_1 , and outer...

The heat flows longitudinally along the length of the tube because the temperature difference is maintained across its flat ends.

The cross-sectional area of the hollow tube through which heat flows is the area of the annular region, given by $A = \pi R_2^2 - \pi R_1^2 = \pi(R_2^2 - R_1^2)$.

The length of the tube is $l$ and the temperature difference is $\Delta T = T_2 - T_1$.

According to the law of thermal conduction, the rate of heat flow is $H = \dfrac{KA\Delta T}{l}$.

Substituting the values, we get $H = \dfrac{K \pi (R_2^2 - R_1^2)(T_2 - T_1)}{l}$.