Question

Is the equation `y=3-x` a direct variation, if so what is the constant of variation?

Answer 1

`y=3-x` is not a direct variation

Explanation:

If `y=f(x)` is a direct variation
then for all values `(x,y)` satisfying `y=f(x)`
`color(white)("XXX")y/x = c` for some constant `c`

Checking a few sample solutions:
`{: (x,color(white)("XX"),y,color(white)("XX"),y/x), (1,color(white)("XX"),2,color(white)("XX"),2), (2,color(white)("XX"),1,color(white)("XX"),1/2), (3,color(white)("XX"),0,color(white)("XX"),0) :}`

Obviously the ratio `y/x` is not a constant for this example
and therefore this is not a direct variation.