Can someone help me with the following question?

A tank contains 100 litres of brine whose concentration is 3grams/litre. Three litres of brine whose concentration is 2 grams/litre flow into the tank each minute and at the same time 3 litres of miture flow out each minute.

(a). Show that the quantity of salt, Q grams, in the tank at any time t is given by:

Q = 200 + 100e^-0.03t

The problem is I get my constant as a negative log.

Thanks.