In the diagram below, points a and b represent two airplanes. how far apart are the airplanes?

Using the cosine theorem we have the following equation:
 d ^ 2 = (800) ^ 2 + (840) ^ 2 - 2 * (800) * (840) * (cos (150-45))
 We cleared d:
 d = root ((800) ^ 2 + (840) ^ 2 - 2 * (800) * (840) * (cos (150-45)))
 d = 1301.327321
 Nearest whole number:
 d = 1301 miles
 
 The airplanes are far about:
 d = 1301 miles

The answer should be c.