ANSWER
21
EXPLANATION
The given by binomial expression is:
![( {x}^(2) + y)^(7)](https://img.qammunity.org/2020/formulas/mathematics/high-school/r0m8c6dld09o02yegvpv8bm8jafxx9awxg.png)
Comparing this to
![(a+ b)^(n)](https://img.qammunity.org/2020/formulas/mathematics/high-school/52kxhkztsq229nnntc17ow3vm03qmvpijn.png)
We have:
![n = 7](https://img.qammunity.org/2020/formulas/mathematics/college/iu68zqnndvqrwplqkwzqjsxaqgbhtowiqk.png)
![a = {x}^(2)](https://img.qammunity.org/2020/formulas/mathematics/high-school/2dio3x0omii439x1jiwnmez1meklxam4pn.png)
![b = y](https://img.qammunity.org/2020/formulas/mathematics/high-school/ielqzbc18qunje8ex4n7be6uqzj2uzcjlk.png)
The coefficient of the (r+1)th term is given by:
![\binom{n}{r}](https://img.qammunity.org/2020/formulas/mathematics/high-school/5sy13zr6w4j401nt5cna37tz816m3thdjb.png)
We want to find the coefficient of the third term:
![r + 1 = 3](https://img.qammunity.org/2020/formulas/mathematics/high-school/f6an5i8qgg5xkvisdt35aiep4y602dhdd7.png)
![r = 2](https://img.qammunity.org/2020/formulas/mathematics/middle-school/t8hdw29mmxpr9yhgs6jtjiwyblt8hrz8k5.png)
Therefore the coefficient is:
![\binom{7}{2} = 21](https://img.qammunity.org/2020/formulas/mathematics/high-school/3glh6i3sqjxxi6gcbaare0kyu7oyykbnix.png)
The coefficient of the third term is 21.