I'll pick a cos graph because that has a maximum at which coincides with the question. The generic formula for such a graph is where A,B,C and D are constants to be found. In this case however since we have a maximum at the start of the graph so there is no offset.

You know that the maximum value is 12m and the minimum 3. Since a cos graph is smooth the middle is the mean of these values: and so

The question had told us that and

Using this information we can say that . To find A and B use the two data points above