Extra practices help her be solid on new concepts.

No, and it should be "in new concepts."

Thank you, Brooke.

The original sentence, "Extra practices help her be solid on new concepts," is grammatically correct.

The preposition "on" is commonly used when talking about being knowledgeable or proficient in a specific topic or skill. In this sentence, "on" is used to indicate that the extra practices help her become solid or proficient in understanding new concepts.

The preposition "in" is typically used to indicate being present within something or being involved in a specific field or domain. In this context, using "in" would imply that she becomes solid or proficient while being actively engaged within the new concept itself, rather than understanding or grasping the concept.

Therefore, the correct preposition to use in this sentence is "on" as it accurately conveys the intention of becoming solid or proficient in understanding new concepts through extra practices.