# sample v for inclusion in parametric equation

I am working through the following equation in the plane-line intersection point. But the question is smaller than this. I am confused about v, the direction vector:

```
t = - (dot(n,p) + d) / dot (n, v)
```

I know one contributing equation is:

```
p = po + t * v
```

I know that v represents the direction vector. What do I calculate v from :

- 2 different points on the plane?
- position vectors on the planes?
- other?