This is the 2584th most frequent Portuguese word.
fraca
Weak.
Here, "fraca" describes the insufficient brightness of the light produced by the lightbulb.
A lâmpada está fraca.
The lightbulb is weak.
In this case, "fraca" refers to lacking proficiency or skill in the subject of mathematics.
Ela é fraca em matemática.
She is weak in mathematics.
Here, "fraco/fraca" is used to indicate a poor or substandard level of performance.