I think the second number is supposed to be negative, -11/2 = -5.5, because otherwise the sequence is not arithmetic.
The <em>n</em>-th term of the sequence, starting with <em>n</em> = 1, is then
-6 + 0.5 (<em>n</em> - 1) = 0.5<em>n</em> - 6.5
and we want to find the number <em>N</em> such that

We have

Solve for <em>N</em> :
0.25 <em>N</em> ² - 6.25<em>N</em> + 25 = 0
<em>N</em> ² - 25<em>N</em> + 100 = 0
(<em>N</em> - 20) (<em>N</em> - 5) = 0
<em>N</em> = 20 <u>or</u> <em>N</em> = 5