Bag a contains 4 red and 3 blue tickets. bag b contains 3 red and 1 blue ticket. a bag is randomly selected by tossing a coin and one ticket is removed from it. using a tree diagram, determine the probability that the ticket chosen is blue.

It's difficult to draw a tree diagram with this software.
Try to do it yourself & you will find the followings:

If A is selected (P(A) =1/2) we can get ether red (p(A & red)=4/7
so P(A∩red)= 1/2 x 4/7 = 4/14
Also we can get P(blue) = 3/7   & P(A∩blue) = 1/2 x 3/7 = 3/14

Same reasoning for B & you will get P(B∩read) 1/2 x 3/4 = 3/8
Also we can get  P(B∩blue) = 1/2 x 1/4 = 1/8
Probability of blues is either 3/14 or 1/8
P(blue) = 3/8 +1/8 =19/56 = 0.339