math

Note that   a_n  is the next term following a_n-1

And

a_n+1 is the next term after a_n

The common difference is  0.4 - 1.2  = -0.8 

And the first term is 1.2  = a₁

So....we have

Next term  =  Previous term - 0.8,  a_n = 1.2

So....the answer that makes sense is

a_n = a_n-1 - 0.8, a1 = 1.2

[ Second answer ]

Does this make sense???