The first term, a is 7.
The next term is increased by adding 7.
The sequence is an Arithmetic Sequence.
The common difference d = 14 - 7 = 21 - 14 = 7
For Arithmetic, an = a + (n - 1)d
The nth term = 7 + (n - 1)7 = 7 + 7(n - 1)
= 7 + 7n - 7 = 7n
= 7n