Unsure how to go about computing GDP for the following problem
Generic growth formula:  

$$\frac{V_t}{V_{t_0}} = (1 + r)^{t-t_0}$$

Solving for $r$ will give you a growth rate in terms of the units of $t$ and $t_0$.  In your example, that's years.  

You currently have a growth rate and the value in 1997.  Projected value in 2012 would be 

$$V_{2012} = V_{1997} \cdot (1 + r)^{2012-1997} = V_{1997} \cdot (1 + r)^{15}$$

This assumes that $r$ is the decimal (not percentage) average growth rate that you calculated.  

I'll leave it up to you whether your original calculations of the growth rate are consistent with this method of calculating the growth rate.  If not, you may want to recalculate it.  

The formula that I gave is a generic one.  While you may be applying it to GDP, this will work for the growth rate of any variable.  That's why I used $V$ for value rather than a variable more commonly recognized as representing GDP.