Explanation

The given sequence is:

3, 4, 7, 8, 11, 12

Form differences between two consecutive numbers in the sequence as follows:

4 - 3 = 1

7 - 4 = 3

8 - 7 = 1

11 - 8 = 3

12 - 11 = 1

X- 12 = 3

= 15