This is an exponential decay function, or it can also be viewed as a geometric sequence, either way, it can be expressed in this circumstance as:
f=ir^t, f=final amount, i=initial amount, r=common ratio or rate, t=times ratio is applied or time.
In this case i=2000 and r=(100-7)/100=0.93 so
f=2000(0.93^t)
We are asked to find when f=600 so:
600=2000(0.93^t) divide both sides by 2000
0.3=0.93^t, take the natural log of both sides
ln0.3=t*ln0.93 divide both sides by ln0.93
(ln0.3)/(ln0.93)=t
t≈16.59
So in about 16.5 years she will own only 600 square miles. (round as appropriate for your answer)