find the direction vector (d) from the normal vector (n).

To find the direction vector, we just need to swap the components of the normal vector and change one of the signs:

d = (-1, -3)

Therefore, the direction vector is (-1, -3).

Does it matter which number you change the sign for?

No, it doesn't matter which component of the normal vector you change the sign for. You can choose either one and you will get a valid direction vector. Swapping the components and changing the sign of one of them is equivalent to rotating the vector 90 degrees clockwise or counterclockwise.

To find the direction vector (d) from a normal vector (n), you can simply switch the components of the normal vector and change the sign of one of them. So, in this case, to find the direction vector (d) from the normal vector (n) n = (3, -1), you would switch the components and change the sign of one of them.

Therefore, the direction vector (d) is (-1, 3).