Let P = {x0,x1,x2, ... ,xn} be a regular partition of the interval [a,b]. (Show that if f is continuous and decreasing on [a,b], then...
Uf(P) - Lf(P) = [f(a) - f(b)]▲x
Notice that this is a regular partition. So $\displaystyle \Delta_x=\frac{b-a}{n}$.
Also this is a decreasing function which means $\displaystyle Uf(P) = \sum\limits_{k = 0}^{n - 1} {f(x_k )\Delta_x }~\&~ Lf(P) = \sum\limits_{k = 1}^n {f(x_k )\Delta_x } $.
Can you see how to finish?