If $I\propto x,y,z$ how can I express a function $I=f(x,y,z)$?

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As the title;

If $I\propto x,y,z$.

How can I express a function $I=f(x,y,z)$?

CA : $I=f(x,y,z)=kxyz$ , for some $k$.

I'm not sure how this answer is derived?