The next number in the sequence is always found by subtracting the number before the previous number from the previous number. So you calculate the number in position 3 by subtracting the number in position 1 from the number in position 2.
Watch out, subtracting a negative number is the same as adding the number.
135 - 15 = 120
120 - 135 = -15
-15 - 120 = -135
-135 - -15 = -120