X = 2 expand the brackets and then collect like terms (x's on one side) and then dived to get x on its own! :)
Answer: So the number of outcomes with exactly 4 tails is 720/2/24 = 15. Finally we can now calculate the probability of getting exactly 4 tails in 6 coin tosses as 15/64 = 0.234 to 3 decimal places.
