The constant of proportionality is 1 although you cannot see it. It is understood to be there.
y = (-8/5) x
This is a nasty piece of work and it depends on your grade level. If you are below grade 8, you are not encouraged to think of a constant of proportionality as less than 0.
In general the opinion is split. Some people say yes others no. If you twist my arm and put a gun to my head, my answer would be that it's fine, but it is a matter of what you have been told. I would read your notes.
