You can find better than google where translations are concerned.
That’s what DeepL gives for “What subject are you good at?“:

Dans quel domaine êtes-vous bon ?

With the options:

• Dans quel domaine es-tu bon ?
• Dans quel domaine es-tu doué ?

Which is correct because we did not give it any context and “subject” can indeed be translated by domaine.

If you give it more context just by adding “at school”, that’s what you get:

Dans quelle matière es-tu bon à l’école ?

The translator has recognised the school context and used matière and the tu form comes first since the question is more likely to be asked to a child or a teenager.

Some people say :

• En quelle matière es-tu bon ?

because that is how the adjectives bon and mauvais are used:

• Je suis bon en maths mais je suis mauvais en français.

You can also choose not to use the word matière :

• En quoi es-tu bon ?

You might also like to read the answer to this question How do you say “He is good at playing” or “I am bad at writing” in French? for other ways to say “good at” and “bad at”.