The explicit rule for a sequence is given.

a₁  = 3(1)  + 1    =  4

a₂  = 3(2) + 1  =    7 =     a₁  + 3

Note that we are adding 3 to every previous term to get the  next term

So

a₁  = 4     and the rule  is   a_n  =  a_n-1 + 3