Given U_{r}= 3/ (r^{2}+3r)

I have simplified^{n}∑_{r=1}U_{r}in terms of n by expressing in partial fraction.

Determine the value of^{∞}∑_{r=1}U_{r}, deduce the value of^{∞}∑_{r=2 }U_{r+1}

I just don’t know how to deduce . Can anyone explain to me ?