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.
54 - 6 = 48
48 - 54 = -6
-6 - 48 = -54
-54 - -6 = -48
-48 - -54 = 6