The given expression fro the sum of n terms:
[tex]\begin{gathered} S_n=\frac{n}{2}(a_1+a_n) \\ \text{ Where: }a_1\text{ is the firm term \& }a_n\text{ is the last term} \end{gathered}[/tex]From the given question we have:
[tex]a_1=8,a_n=79,\text{ n =6}[/tex]Substitute these value in the expression of sum of n terms
[tex]\begin{gathered} S_n=\frac{n}{2}(a_1+a_n) \\ S_6=\frac{6}{2}(8+79) \\ S_6=3(87) \\ S_6=3\times87 \\ S_6=261 \end{gathered}[/tex]So, sum of 6 terms is 261
