The summation symbol means that we need to add the terms (n-1) for n = 1, 2, 3, and 4.
In other words, we have the following terms:
[tex]\begin{gathered} 1-1=0 \\ \\ 2-1=1 \\ \\ 3-1=2 \\ \\ 4-1=3 \end{gathered}[/tex]
And we need to add them:
[tex]0+1+2+3=6[/tex]
Therefore, the result is 6.
