How do you factor 30n^2b-87nb+30b?

It's not clear whether you meant 30n^ (2b) or [30n^2] b.

I will assume the latter.

b can then be factored out of all three terms, resulting in

b [30n^2 - 87n + 30] = b [ 3 ] [10n^2 - 29n + 10 ]

Can you factor this further?