Before I can answer this question, there’s two things you need to know about MOST teenagers: 1) They want the easiest path possible. 2) They will manipulate the truth for a favorable outcome. If you feel these aren’t true for you child then, congratulations, you truly have a unique teenager!
So the first answer to the original question? Help them find their motivation!!! Its impossible to convince a teenager (and most adults) that math is important when they already have it in their head that its not. Don’t even try. Instead motivate them from the standpoint that doing well in school opens doors that will otherwise close. The truth is that students that struggle mathematically often end up taking the most math classes. It can get really frustrating really fast.
The second honest answer? Push and support. Push them to do their best and support them with bumps along the way. Remember that this a marathon not a sprint. There are no quick fixes. A student struggling in math either lacks the skills to solve the problem or the understanding behind the concept. Hard work can help with both of things but wanting to work hard comes from motivation. You can see the symbiotic relationship we have here.