@shairnn So here is two case in which 2nd one is most logical.
Case 1: Multiple with previous number so down the line we get 2 x 3 = 6.
Case 2: if we only use the num given in ques
Patter is multiplying prev num with 2 and sub in the result above so ans we get is 20-(3 x 2) = 20 - 6 = 14.