Question 180660
Start with your equation.
{{{C= 344.3t + 5400}}} for {{{6<=t<=11}}}
Find t when C=11000.
You assume that the equation stays linear when {{{t>11}}}.
{{{344.3t + 5400=11000}}}
{{{344.3t=5600}}}
{{{t=16.3}}}
Since t=6 is the year 1996, then year t=16 would be year 2006. 
The leftover 0.3 year would get you to roughly 3rd or 4th month of 2006. 
I'm not sure if you need to be that exact or if you can say between 2006 and 2007.