Wyzant - A source for lessons by topic, and online tutoring / helpdesk support if needed also!
Wolfram Alpha - Wonderful computation engine, will run calculations, can graph and interpret functions.
Interact Math - A website with practice problems from the Demana PreCalculus textbook.
Determinants of 3x3 matrices - This link will show, step-by-step, how to find the determinant of a 3x3 matrix. Check it out!