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.
156 - 13 = 143
143 - 156 = -13
-13 - 143 = -156
-156 - -13 = -143
-143 - -156 = 13