Your body loses 6.8 kJ of heat when 2.8 g of sweat evaporates from your skin at 25 °C.
This is a problem that involves phase changes. The sweat is changing from the liquid phase to the gaseous phase.
It takes energy to separate the liquid molecules from each other and get them into the gas phase. The sweat is evaporating. The energy required is the enthalpy of vaporization
The enthalpy of vaporization of water varies with temperature. At 25 °C,