What we have here is a geometric progression, because the common ratio is -3.

The n^{th} term of a geometric sequence can be found by using the formula a_{n} = ar^{n-1}

where a is the first term. r is the common ratio, which is -3.

Plugging these values in the above formula we get,

So the 20th term or a_{20}=2\times\left(-3\right)^{20-1}=2\times\left(-3\right)^{19}=-2,324,522,934

I found an answer from math.stackexchange.com

18th derivative of $\arctan(x^2)$ at point $x=0$ - Mathematics Stack ...

Hint. One may write d18dx18arctan(x2)=d17dx172x1+x4. then, by a partial fraction decomposition, one has 2x1+x4=12ℜ(ix−1−i√2)−12ℜ(ix−1+i√2). Then using.

For more information, see 18th derivative of $\arctan(x^2)$ at point $x=0$ - Mathematics Stack ...

I found an answer from news.yahoo.com

93-year-old Florida mayor prepares run for 20th term in office

Dec 20, 2013 ... By Barbara Liston ORLANDO, Florida (Reuters) - A 93-year-old Florida man believed to be the country's oldest mayor is seeking re-election ...

For more information, see 93-year-old Florida mayor prepares run for 20th term in office