Can someone explain to me the steps to this problem in detail? For example, in the photo of the 2 circles, where did the 2&4 come from? Why is P(A and B) = 3?

Step-by-step explanation:
If you want to find the probability of union of two overlapping events,
when you add the probabilities of both the events, that overlapping part gets added two times, so you gotta subtract it.
P(A and B) = 3 is incorrect. It should be P(A and B) = 1/6
This is because there is one number in the overlapping region, and this is out of 6 numbers total.
The 2 & 4 are values that do not fit either event A or event B, such that
event A = rolling an odd number = {1, 3, 5}
event B = rolling a number that is a multiple of 3 = {3, 6}
however, the values 2 & 4 are still found on the die that is rolled, so they are listed outside the two venn diagram circles.