you can just use the word 不，it's negative. The sentence 我不擅长数学 has the same meaning of I am bad at math. using the 不 is simple . 拙于 is not in so common use I think.

Actually, usually , We don't say “擅长”。 In most case, "擅长" use to ask somebody or Appear in your resume. such as A：你擅长做什么工作？ Q：销售。 or 我做销售比较拿手。 I'm good at math , we say "我数学很好"commonly。 I'm bad at math , we say “我数学不好 or 我数学很差、我数学不行” natural Chinese

The literally translation for "I'm not good at mathematics" is “我不擅长于数学”。 but for "I'm bad at mathematic", you can also say "我数学很差“ [My mathematics is bad], which is more common in spoken Chinese.

Ha ha, let me tell you a high level expression. That's “我拙于数学”means “i'm bad at mathematics”.