Which is a well known seperable equation. In this case, I imagine you don't need to derive the equation so we'll skip right to it.
We are going to use
where we let t be time in hours.
At time 0 we have N bacteria so the above equation stays the same.
At time 2 we have 4N bacteria so we can find K,
Now, we sub back into our first equation to get.
At what time t does the colonie have 10 bacteria?