Answer:

And we can set equal this derivate to 0 in order to find the critical point and we got:


And we can calculate the second derivate and we got:

So then w can conclude that the value of t = 3.4375 represent the minimum value for the function and we can replace in the original function and we got:

So then the minimum annual income occurs at t = 3.43 (between 2008 and 2009) and the value is 25.094
Step-by-step explanation:
For this case we have the following function:

Where P represent the annual net income for the period 2007-2011 and 
And t represent the time in years since the start of 2005
In order to find the lowet income we need to use the derivate, given by:

And we can set equal this derivate to 0 in order to find the critical point and we got:


And we can calculate the second derivate and we got:

So then w can conclude that the value of t = 3.4375 represent the minimum value for the function and we can replace in the original function and we got:

So then the minimum annual income occurs at t = 3.43 (between 2008 and 2009) and the value is 25.094