Sometimes failure happens.  Here's how you can recover credit.  

Take a Credit Recovery class period at Westlake

This option is free and would require you to give up an elective (or off period) and be placed in the class during the regular school year. The credit recovery program is computer-based and self-paced.  You can be enrolled for a semester or for a year.  Talk to your counselor about putting this class in your schedule.

Enroll in Westlake's summer school program

There are two sessions (one in June and one in July); classes meet Monday-Thursday.  Find information here (this site is typically updated by Spring Break of every year).  There is a cost to attend summer school.

Enroll in an online correspondence course

Westlake accepts credits from Texas Tech and UT high school programs.  These online courses are self-paced (though they do have an expiration date), and final exam is required which is also completed online; there is a fee.  Learn more here (top half of the page).

Take a credit-by-exam (CBE)

If you think you have a decent understanding of the subject, but failed because you didn't turn work in on time or didn't put in much effort, you might want to try a CBE.  You would take the exam, and as long as you make a 70 or better, you earn credit.  This can be done through Texas Tech or UT high school programs (the exam is proctored online); there is a fee. Learn more here (bottom half of the page).