X + y + 2xy = 21; 2x^2 + 2y^2 - xy = 7

It looks like you have a set of equations that you want to solve or analyze. Let's go through them one by one:

### a)
1. **Equations:**
\( x + y + 2xy = 21 \)
\( 2x^2 + 2y^2 - xy = 7 \)

### b)
2. **Equations:**
\( x^3 + y^3 + x^3 y^3 = 17 \)
\( x + xy + y = 5 \)

### c)
3. **Equations:**
\( x + y + x^2 + y^2 = 8 \)
\( x^2 y^2 + xy = 12 - xy(x + y) \)

If you need help with a specific part of these equations, like solving them or simplifying, please let me know!