This could be confusing so I'll explain. What you need to do is to do the inverse so in this case you would multiply the v on both sides. The v's would cancel out and the one times v will be v due to the identity property of multiplication, and that will leave you with but I personally don't like that so I flip it around to .
This is the concept of differential equations, given that dp/dt=rp then: dp/p=rdt thus: ln p=rt+C p=e^(rt+C) P=Ke^(rt) when t=210 days=7 months, p=2k 2k=k×e^(7r) 2=e^(7r) ln2=7r r=ln(2)/7 r=0.0990