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A lot depends on how much competition there is for those keywords. If you're not ranking, it's either because you haven't (a) given it enough time to see your rankings go up, or (b) you haven't overcome the strength of your competition for those keywords. First I would take a look at the pages you are trying to rank for and make sure that they are highly relevant for your keywords, contain helpful information for your target audience, and are optimized onsite (on-page SEO). Then I would just focus on creating link-worthy content around your keywords and post on social media and other blogging sites, sharing your link back to your main content. In turn, you'll hopefully get others to share your content, helping increase the number of backlinks that are pointing to the pages you are trying to rank. Just remember to focus on user experience and optimize for both your users and the search engines, not just the search engines. I hope this helps.

I would go with a sub-folder if passing DA is your goal. As far as I understand it, DA is not considered by Moz for sub-domains. Here is a similar post on the same topic. I hope it helps.

https://mza.bundledseo.com/community/q/is-moz-da-inherited-from-tld-for-all-subdomains-on-the-tld https://mza.bundledseo.com/community/q/is-there-a-way-to-get-the-domain-authority-of-a-subdomain

I would go with a sub-folder if passing DA is your goal. As far as I understand it, DA is not considered by Moz for sub-domains. Here is a similar post on the same topic. I hope it helps.

https://mza.bundledseo.com/community/q/is-moz-da-inherited-from-tld-for-all-subdomains-on-the-tld https://mza.bundledseo.com/community/q/is-there-a-way-to-get-the-domain-authority-of-a-subdomain

A lot depends on how much competition there is for those keywords. If you're not ranking, it's either because you haven't (a) given it enough time to see your rankings go up, or (b) you haven't overcome the strength of your competition for those keywords. First I would take a look at the pages you are trying to rank for and make sure that they are highly relevant for your keywords, contain helpful information for your target audience, and are optimized onsite (on-page SEO). Then I would just focus on creating link-worthy content around your keywords and post on social media and other blogging sites, sharing your link back to your main content. In turn, you'll hopefully get others to share your content, helping increase the number of backlinks that are pointing to the pages you are trying to rank. Just remember to focus on user experience and optimize for both your users and the search engines, not just the search engines. I hope this helps.