angle of depression from a to fire is 90-30=60 from east-west horizontal
angle of depression from b to fire is 90-42=48 from east-west horizontal
So the angle from fire adjacent to the east-west horizontal is 180-60-48=72
Now we can use the law of sines to solve for the distances...
sin72/12mi=sin48/A=sin60/B
A=12sin48/sin72=9.38mi
B=12sin60/sin72=10.93mi