``` 13) {x - y = 6 {x³ - y³ = 126 14) {x³ - y³ = 3(x - y) {x + y = -1 15) {2x + 3y = 5 {3x² - y² + 2y = 4 ```

----- Nội dung ảnh -----
```
13)
{x - y = 6
{x³ - y³ = 126

14)
{x³ - y³ = 3(x - y)
{x + y = -1

15)
{2x + 3y = 5
{3x² - y² + 2y = 4
```