If two random variables have the joint density find the probabilities that

Question 6 answer is given under the first image (A=1/4 and B=1/24). I dont need to solution of question 6, just need to solution of question 7 which is included second image.

Q6) If two random variables have the joint density f(x1,x2)={x1x20for01, and F(x1,x2)=1 for x12 and x21 : F1(x1)=0 for x10,1(x1)=41x12 for 0