sol = ode45(...) replies a struct, as the error message tells clearly. Either use:
Or use two outputs
You obtain xc from t=0 to 100, but the 2nd integration goes from 0 to 20. Are you sure that you can add them directly? Is this meaningful?
You provide an initial with 3 elements in:
[t, xqxc] = ode45(F,[0 100], [0 0 0]);
But F replies a [2 x 1] vector only.