Trying to check subscripts

I am writing an activity to help students learn about sequence notation. In screens 2 & 5 they have to type subscripts that are binomial. I believe the latex is correct but it isn’t being checked that way. Can someone help me out? Here is the work-in-progress:

It may be due to the graphing calculator not allowing - in a subscript (i.e. you get an error).

I was surprised! It needs


instead of

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Thank you so much! I always forget to check that. It seems so random.