Linear sequences increase or decrease by the same amount each time. In the diagram above, it is clear that the first sequence is going up in steps of 2. You can even see the straight line it creates. This is why this is a linear sequence.
The first column gives the first term in the sequence = 1 x2 = 2
The second column gives the second term in the sequence = 2 x2 = 4
The third column gives the third term in the sequence = 3 x2 = 6
and so on...
From this, we can recognise that any number x2 is equal to the number in the sequence.
e.g. the 100th term (number in this sequence) = 100 x 2 = 200
the nth term in this sequence = n x 2 = 2n
In the second diagram, the nth term still relates to 2n as the sequence is increasing by 2 each time, but every term is one more than the two times tables so one more is added to each term (see my pink ducks).
Therefore, th nth term for this sequence
= 2n + 1