"She is bad at math."

Translation: Matematika jí jde špatně.

[OndrejSaska]

What about "Matematika jí nejde"?

[endless_sleeper]

It's fine.

[cptlangstrumpf]

Would it be possible to use nedobre instead of spatne?

[kacenka9]

Not really. People would understand what you mean but it is not a used word.

[ValaCZE]

No. Nedobře is very rarely used and it is mostly connected to describe someones wellbeing.

[klyde534622]

Is this correct? "Ji nejde matematika"

[endless_sleeper]

No. You need 'jí' (with an accent) and it needs to be the second word: 'Nejde jí matematika.' Short 'ji' is used only in accusative, you need dative here.

[logica]

Non-linguistic remark: Great that Matej loves mathemaics, but a pity that she is bad at it. Maybe add a sentence that Katerina or Zofie or one of their daughters loves mathematics?

(I am a mathematician so I don't want to put anybody off math and hope that by good teaching, everyone will come to love it.)

[VladaFu]

That is a good idea, because "Matematiku má ráda i Žofie.".