Can you explain the this example

Example 11.1 The heat of mixing for liquid species 1 and 2 at fixed T and P, measured calorimetrically, is represented in some appropriate units by the equation H=x1x2(40x1+20x2) Determine expressions for H1E and H2E as functions of x1. Solution 11.1 We first recognize that the heat of mixing is equal to the excess enthalpy of the mixture [Eq. (11.7)], so we have HE=x1x2(40x1+20x2) Elimination of x2 in favor of x1 and differentiation of the result provides the two equations: 11.2. Heat Effects of Mixing Processes Substitution into both Eqs. (11.14) and (11.15) with HE replacing ME leads to: H1E=2060x12+40x13andH2E=40x13