As you can see, the sequence starts at 3 and increases by 2.5 units each time, so a possible formula for the sequence is:
an = 1/2(5n + 1)
let's verify it:
a1 = 1/2(5*1 + 1) = 1/2(6) = 3
a2 = 1/2(5*2 + 1) = 1/2(11) = 5.5
a3 = 1/2(5*3 + 1) = 1/2(16) = 8
a4 = 1/2(5*4 + 1) = 1/2(21) = 10.5
