You don't say what it is, but I think that h is the half life, 1690. In that case, the formula h = ln2/k allows you to calculate the value of k.

However, why bother to remember one more formula when you don't have to. Better, (from the point of view of not cluttering your memory with formulae you don't need), is to work from the basic formula That way, you get a better understanding of what is going on.

You're told that the half life is 1690 years, so

(This is the equation that gives rise to 'the other' equation h = ln2/k).

Solve it for k.

When there are 5 gram remaining,

Solve this for t, substituting for k on the way.