Zij is goed in wiskunde. (She is good at maths.)
goed takes "in" (goed in = good at). Example: Zij is goed in wiskunde. (She is good at maths.)
Zij is goed in wiskunde. — She is good at maths. The preposition in is fixed, so it doesn't change with the sentence.
Fixed prepositions come up all over the A2 exam. The platform drills them in real exam tasks, AI-graded across all five DUO sections.