Question

insert one comma to separate the coordinate adjectives. although lings upper-level algebra class requires long hours of studying, she is excited about the course because she enjoys solving advanced complicated math problems.

Explanation:

Coordinate adjectives are adjectives that modify the same noun and can be separated by "and" (or a comma). Here, "advanced" and "complicated" both modify "math problems" and are coordinate adjectives. So we insert a comma between them.

Answer:

Although Ling's upper-level algebra class requires long hours of studying, she is excited about the course because she enjoys solving advanced, complicated math problems.