We have:
[tex] a_1=\boxed{-1}\\\\n=\boxed{7}\\\\d=a_2-a_1=2-(-1)=2+1=\boxed{3} [/tex]
so:
[tex] S_n=\dfrac{2a_1+(n-1)d}{2}\cdot n\\\\\\S_7=\dfrac{2\cdot(-1)+(7-1)\cdot3}{2}\cdot 7=\dfrac{-2+6\cdot3}{2}\cdot7=\dfrac{-2+18}{2}\cdot7=\dfrac{16}{2}\cdot7=\\\\\\=8\cdot7=\boxed{56} [/tex]
