The 15th term of the sequence would be -70.
To find this, you must first found out the change between each number in the sequence. Simply subtract the second term, by the first term, to show that each number is being subtracted by 6.
Now, you could simply subtract 6 from -4, and each outcome after that until you get to the 15th term. The numbers in the sequence to the 15th term are...
14, 8, 2, -4, -10, -16, -22, -28, -34, -40, -46, -52, -58, -64, -70.
I hope this helps!