First things first: When you want to indicate a subscript in plain text, use an underscore, thus: a_3 means . If there is more than one character in the subscript, then enclose the subscript in curly braces, thus: a_{10} means . You don't say so (and you most certainly should have) but I will assume an arithmetic sequence.
From to is two terms of the sequence. The difference between the two terms given is . Since that is the difference between terms 10 and 12, the common difference for the sequence must be half of that, or . The number of terms from to is . Then
So:
John
My calculator said it, I believe it, that settles it
From
I > Ø
