6, 11, 16 are three A.Ms between (a) and (b).

Sequence: (a,\ 6,\ 11,\ 16,\ b) (5 terms)

[ d = 11 - 6 = 5 ]

[ 6 = a + d \implies a = 1 ] [ b = 16 + d = 21 ]

Answer: (a = 1,\ b = 21)