?,6,13,20,27 ?7,14,21,28 Pertanyaan brp nilai tanda ?

mis : ? = x

maka
x,6,13,20,27
=> U3= 13
=> U2 = 6
b = 7
maka
U1 = x = 6-7 = -1

x,7,14,21,28
=> U3 = 14
=> U2 = 7
=> b = 14-7= 7
U1 = x = 7-7 = 0