I'm not sure where the -310 or +400 came from, but somewhere in your solving of it something went wrong, because those should be +70 and +20 respectively. My guess is that something went wrong with the +C part? Remember that when you have a ln on one side of the equation and you want to turn it into an exponential, the *entire* side of the equation goes into the exponent, which will include the +C part.