Help with steady-state unemployment

Hello,

I am a first year econ student, trying to work through Mankiw's Macroeconomics 7th edition textbook. However, I have run into a problem that I cannot seem to find where to start.

The question is: The steady-state rate of unemployment is U/L = s/(s+f). Suppose that the unemployment rate does not begin at this level. Show that unemployment will evolve over time and reach this steady state. (Hint: Express the change in the number of unemployed as a function of s, f, and U. Then show that if unemployment is above the natural rate (or the steady-state rate), unemployment falls, and if unemployment is above the natural rate unemployment rises.)

If anyone can point me in the right direction, I would greatly appreciate it.

Thanks.