Let the complete math problem be represented by 1
Brie completed 4/9 of her math problems after school. It means that the portion completed after school is 4/9 * 1 = 4/9
She completed 1/3 of her math problems in the evening. It means that the portion completed in the evening is 1/3 * 1 = 1/3
The portion completed after school and in the evening is
4/9 + 1/3 = 7/9
The fraction of her math problems that is completed is 7/9