Not understanding

The **x** intercept of a line is the value of **x** at the point where the line intercepts the x-axis.

It can be found by setting the value of **y** to **0** and solving for
**x**.

So in this case **x** = -21.

The y intercept of a line is the value of y at the point where the line intercepts the y-axis.

It can be found by setting the value of x to 0 and solving for y.

So in this case 7**y** = -21 and **y** = -3.

Horizontal and vertical lines are special cases where this definition and method has some differences.