We had one similar the other day.....but, anyway....

AB and AO are legs of right triangle AOB with AB = 17000 and AO = 4000

And BO is the hypotenuse.....so

BO = sqrt ( 17000^2 + 4000^2 ) ≈ 17464 miles

But.....we only want BC which = BO - OC = 17464 - 4000 = 13464 miles