A formula is expressed as D = a (2+kt). Express k in terms of D, a, and t.

This is just rearranging terms, it gets kind of ugly though. Start by dividing both sides by a to get D/a=2+kt, next move the 2 over to the left to get (D/a) - 2=kt, finally divide the whole expression by t to get ((D/a) - 2) / t=k