Calculus - Parametric Equations

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I thought that I could convert easily between cartesian and parametric equations but apparently I was wrong. I have the equation y=12/x and when converting it to parametric I thought I could just say y=12/t and x=t I don't see why this wouldn't work because if you go from parametric to cartesian it works just fine. Any help putting y=12/x into parametric form?