if she worked "x" hours lifeguarding at $13 per hour, that means she earned a total of 13*x or 13x, if she worked washing cars for "y" hours at $12 per hour that means she made on those hours 12*y or 12y, and we know that whatever their sum combined is a grand total of $150.
Last week she worked a total of 12 hours doing both, namely x + y = 12.