Hi Justin,
Thanks for writing in. The optional essay can be a good place to address such weaknesses as it demonstrates you are aware of the weakness and allows you to take control of the issue, as opposed to hoping the admissions office notices those counterpoints on your application.
If you choose to write this optional essay I advise you keep it to one paragraph (3-5 sentences) and avoid trying to justify your low score. Rather just plainly state that although your quant score is lower than they may like to see, you are confident you can handle MBA-level math due to your strong undergraduate performance, work experience, etc.
Best of luck!
Daniel Richards
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