0, 3, 8, 15, 24, 35, 48, 63,...

 0 =  1-1 = 1²-1
 3 =  4-1 = 2²-1
 8 =  9-1 = 3²-1
15 = 16-1 = 4²-1
24 = 25-1 = 5²-1
35 = 36-1 = 6²-1
48 - 49-1 = 7²-1
63 = 64-1 = 8²-1

Look at that list on the right

Any term is that term's number minus one.

the 4th term is 4 squared minus 1

the 7th term is 7 squared minus 1 

So the 80th term is 80 squared minus 1 or 6400-1 = 6399.

Edwin