Solution

In[33]:=

(*1*)x = {1, 2, 3, 5, 7, 11}

Out[33]=

{1, 2, 3, 5, 7, 11}

In[34]:=

(*2*)x[[1]] + x[[6]]

Out[34]=

12

Another way to do the same thing is with the functions First and Last

In[35]:=

First[x] + Last[x]

Out[35]=

12

In[36]:=

(*3*)x[[2]] * x[[3]] + x[[4]]

Out[36]=

11

In[37]:=

Clear[x]

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