Assuming that flux density are as in Exercise 5 and that
and
v are
positive constants, use a conservation law argument to derive (a) a centered
explicit FDE
of the form
and (b) an ``upwind'' explicit FDE of the form
Formulate similar FDEs for the case that 
and v are allowed to
depend on x and t; for
the case that 
and v are allowed to depend on u.
