A3 = 81 and a6 =648 what is the 1st term?

The answer depends on what you know about the sequence ...

If is geometric, then

for some fixed number r. Using this recursive rule, we get

and so on, down to the 1st term in the sequence:

This means

so that

Now solve for :

If instead is arithmetic, then

for some fixed number d. Similarly, we can get the n-th number in the sequence in terms of the 1st one:

and so on, down to

Then

Solve for d :

Now solve for :

the translated point is (5,2)