Answer
The increase in the thermal energy of the bed and floor $=3.5$ $J$
Work Step by Step
The increase in the thermal energy of the bed and floor is equal to the work done by the kinetic frictional force.
So,
frictional work = kinetic frictional force $\times$ displacement of box
$= 5N\times 0.7m$
$= 3.5$ $Joules$
