Chapter 11 - Section 11.4 - Partial Sums of Arithmetic and Geometric Sequences - Exercise Set - Page 658: 2

$S_7=-133$

Using $S_n=\dfrac{n}{2}(2a_1+(n-1)d)$, the sum of the first $ 7$ terms of the sequence $ -7,-11,-15,... $ is \begin{array}{l} S_7=\dfrac{7}{2}(2\cdot(-7)+(7-1)(-4))\\\\ S_7=\dfrac{7}{2}(-14-24)\\\\ S_7=\dfrac{7}{2}(-38)\\\\ S_7=-133 .\end{array}