Question: How can I calculate F(6/7) - F(-6/7) to find probability related to central limit theory?

How can I calculate F(6/7) - F(-6/7) to find probability related to central limit theory?

I need to first use the F:=x-->y to define F. What I'm getting is not helping me answer my question. could someone please tell me the steps to find an answer? 

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