Alaska Airlines is the fifth largest airline in the United States. Its headquarters are at the SeaTac (Seattle Tacoma) airport in Washington state. Its fleet size in 2019 was 336 planes.
N251AK is a Boeing B737-900 extended range (ER) which was delivered to Alaska Airlines in 2016.
Picture from Los Angeles LAX airport 26.10.2016 by Ilkka Siissalo.

N253AK is a similar Boeing B737 version 900ER plane like the one shown above.
Picture from Los Angeles LAX airport 29.10.2016 by Ilkka Siissalo.

A similar B737 jet like the ones shown above, but in a newer livery of Alaska Airlines.
Picture from Los Angeles LAX airport 29.10.2016 by Ilkka Siissalo.

A second view of the same B737 as above.
Picture from Los Angeles LAX airport 29.10.2016 by Ilkka Siissalo.