13-year-old homeschool student struggling in Algebra I

I have an 8th grader who is taking Algebra I through the online homeschool curriculum, Teaching Textbooks.  She has always excelled in Math, even in TT's Pre-Algebra course last year, but is now consistently struggling with each Algebra I lesson and is only about 1/5 of the way through the course.  I think the best thing for her to do is to take a break from the TT Algebra I course and see if any of the Khan Academy courses can help.  My big question is... how do I know what KA course to have her do?  Should I have her do KA's Pre-Algebra or Algebra Basics course?  I'm assuming one of those two would be the best choice, but are there other options we should consider?  Also, how long does it take to complete one of those courses?  It is now January, so I'm hoping it would not take a full school year.  Any advice or recommendations would be greatly appreciated!