If the area of the sector is bounded by a 70° arc then that is also the measure of the central angle in question. The formula for the area of a sector is

. We now have our angle beta and we were given the radius, so filling in accordingly, we have

which simplifies to

. If you have to leave your answer in terms of pi, then you're done. If you have to multiply in the value for pi, you would have 137.44 miles squared.