Geometry is a subject she's obviously having great difficulty with; perhaps a creative private tutor could find ways to help her "get it". I'm not blaming her current geometry teacher for her present grade but I am saying that oftentimes a different approach to learning a particular subject can give a child the new tools she needs to be a confident learner. You can also find out what her teachers think her strengths and her areas that could be improved are. Don't just wait for grades to come back to get involved.
Have a few non-pressured talks with her about what she thinks needs to happen for her to do her best in school. Don't judge her or blame her for her grades. Really make an effort to let her know you understand how tough it is to be a 15-year-old and that you know there are lots of other important things going on in her life besides school. Let her know you believe in her and that you want her to do the best she can, whatever that may be. Offer her the option of tutored help. Also praise her for any efforts she is making, not just for actually getting a better grade. We all like to be appreciated and acknowledged for our sincere efforts, not just the results of those efforts. You also may look to find non-academic accomplishments to praise her for. Sometimes even noticing something so trivial as the fact that she has cleaned up after herself better in the kitchen, can have a ripple effect in making her feel you notice when she's trying.