def METHOD_NAME(ctx, C):
    """
    Compute the third term of the equation described on p. 1144:
    ``(s11 + s12 * (M - 6)) * Sc + (s21 + s22 * (M - 6)) * Sd`
    """
    Sc, Sd = _get_site_type_dummy_variables(ctx)
    return (C["s11"] + C["s12"] * (ctx.mag - 6.0)) * Sc +\
        (C["s21"] + C["s22"] * (ctx.mag - 6.0)) * Sd