Please help me on this calculus/related rates problem?

Let f(t) be the weight (in grams) of a solid sitting in a beaker of water. Suppose that the solid dissolves in such a way that the rate of change (in grams/min) of the weight of the solid at any time t can be determined from the weight using the formula: f ' (t) = -3 f(t) (1+ f(t))......If there is 4 grams of solid at time t=2 , what is the amount of solid 1 second later?