Answer:
He would pay $287 if he races 39 times
Thus, the membership fee is $14.
Explanation:
Linear Modeling
The situation described in the problem can be modeled as the equation of a line which has the form:

Where y is the cost of the course, x is the number of races, a is the cost per race, and b is the membership fee.
We know the course charges $7 per race, so a=7. Since your friend has raced x=28 times and paid y=210, we can find the value of b:

Operating:

b=14
Thus, the membership fee is $14.
The linear model is now:

If he races x=39 times, your friend would pay:

y=287
He would pay $287 if he races 39 times