Alex usually runs for 21 hours a week. We know that there are 7 days in a week. 21/7 = 3, which means that Alex runs for 3 hours each day.
We are trying to figure out how much time he loses if he is unable to run for three days. 3 * 3 = 9. This means he loses 9 hours of training while he is unable to run.
Assuming he runs the same number of hours per day, then he runs 3 hours per day (21/7). If he runs 3 hours a day, then in 3 days he would have ran 9 hours (3*3).
He loses 9 hours of training if he doesn't run for 3 days.
The coefficient of a variable is simply the number it's being multiplied by or the number that is in front of it. In this case, the coefficient of y is 7.
