She has 7 classes a day (3+4), and each takes the same number of minutes. If m stands for minutes per class, you can find the total minutes per day by multiplying 7 by m. Therefore the answer is 7m.
Example: Let's say a class takes 30 minutes, you would put 30 in the place of m, 7x30=210, so she would be teaching 210 minutes of yoga class each day.
