Two Planes leave an airport at the same time, one flying 300km/h and the other at 420km/h

The angle between their flight paths is 75 degrees after three hours, how fart apart are they?
answer:
so it would be c^2=a^2+b^2-2ab cos C
900^2+1260^2-2(900)(1260)(cos75)
c^2=33542.9
c=183.14km
is it right?

Right equation, but you did not do it correctly.

c^2 = 810,000 + 1,587,600 - 587,002
= 1,809,998
c = 1345 km