excel 2003 automatically paste with increment

[filnirv]

Hi friends,

Could you help me with a problem that i have in excel 2003.
Imagine that i have:
a1=b3
a2=b10
a3=b17

so we have an increment of 7 cells. Is it possible to make automatically for a4, a5, a6 with the same increment or different increment?

Thanks for your help
Filnirv