The wording of this question is a bit confusing... You can't write a sequence in sigma notation, but rather a series or sum. I think the question is asking you to write the sum of the sequence,
[tex]\dfrac12,\dfrac14,\dfrac18,\ldots,\dfrac1{64}[/tex]
which would be
[tex]\dfrac12+\dfrac14+\dfrac18+\cdots+\dfrac1{64}[/tex]
in sigma notation.
To do this, notice that the denominator in each term is a power of 2, starting with [tex]2^1=2[/tex] and ending with [tex]2^6=64[/tex]. So in sigma notation, this series is
[tex]\displaystyle\sum_{n=1}^6\frac1{2^n}[/tex]
