[college math] Finding the missing unit in an equation.

I have this equation: d/t = d/t - md - d/(t/i) where d, t, and i are measuring units. I need to figure out the value of m as it is a measuring unit but I don't know what it's supposed to hold. For example, it can be d/t, simply d, etc. I went as far as to get to this point:

(d/t)-(d/t) = -md - di/t

md = -di/t

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But I don't know where to go from there or if I took the right steps... I resorted to some videos regarding figuring out the missing measuring unit but I'm unable to apply them here.

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This equation is used for an ecology model btw but I didn't think it was relevant to explain it, I just need to find the measuring unit of m.