1. I have to do scince and math homework tonight.
2. I have to do my science and math homework.
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Are both grammatical? Which one is commonly used? Do you have other way of expressing?
Both are correct (fix the typo "science" in #1), and both are commonly used.
Both sentences are grammatical and can be used interchangeably depending on the context and personal preference.
However, the second sentence "I have to do my science and math homework" is more commonly used. It clearly states that the speaker needs to complete their homework in both science and math subjects.
Another way to express the same idea could be "I have to complete my science and math assignments." This sentence uses the word "assignments" instead of "homework" to convey the same meaning.