To find the factor tree of a number , break the number to a pair of factors. Here 48 = 6 x 8
Continue the process till you get prime numbers as factors. So we get 6 = 2 x 3 and 8 = 2 x 2 x 2
And your answer is the product of all these prime factors.
i.e., 48 = 2 x 3 x 2 x 2 x 2