We are given with an AP (Arithmetic Progression) with first term being -7, and the common difference being the difference between any succeding term from any term and the term itself, so common difference = -11 - (-7) = -11 + 7 = -4, so now before finding the sum of 21 terms, let's recall that ;
- [tex]{\boxed{\bf{S_{n}=\dfrac{n}{2}\{2a+(n-1)d\}}}}[/tex]
Where, d is the common difference, a is the first term, and n is the number of terms and [tex]{\bf S_n}[/tex], being the sum of n terms, so putting all the values in the formula, we will have ;
[tex]{:\implies \quad \sf S_{21}=\dfrac{21}{2}\{2\times -7+(21-1)-4\}}[/tex]
[tex]{:\implies \quad \sf S_{21}=\dfrac{21}{2}(-8+20\times -4)}[/tex]
[tex]{:\implies \quad \sf S_{21}=\dfrac{21(-8-80)}{2}}[/tex]
[tex]{:\implies \quad \sf S_{21}=\dfrac{21(-88)}{2}}[/tex]
[tex]{:\implies \quad \sf S_{21}=-21\times 44}[/tex]
[tex]{:\implies \quad \boxed{\bf{S_{21}=-924}}}[/tex]
