Hey guys, just a bit of help please.

The first term of an arithmetic sequence is a. The mth term is zero. Find the rule for tn for the sequence.

so far i got

a=a+d(n-1)

0=a+d(m-1)

Subtract:

a=d(n-1)-d(m-1)

...

d=a/(n-m)

so the rule i would write: t_n=a+a/(n-m)

but this isn't right - can anyone see where i went wrong?