Is x+3y=24 perpendicular, parallel, or neither?

You 1st need to change that line into y=mx+b, then you can see what the slope is, once you have done that you will need to find it's inverse which you will do by following these steps:
1. change f(x) to y
2.swap x for y
3. solve for y
once that is done you will be able to see the slope of the inverse and when you compare it to the original line, they are the same, therefore parallel.