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:
![y=ax+b](https://img.qammunity.org/2021/formulas/mathematics/middle-school/rkutszlajtnwcvo050qia6xch8w6a7ac8c.png)
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:
![210=7(28)+b](https://img.qammunity.org/2021/formulas/mathematics/college/rfadyy05qre8wa7dhg2kof87m72nyvrbtv.png)
Operating:
![210=196+b](https://img.qammunity.org/2021/formulas/mathematics/college/zrjuf7nib4pm6uuh3gd46t7q5mjh5rru8t.png)
b=14
Thus, the membership fee is $14.
The linear model is now:
![y=7x+14](https://img.qammunity.org/2021/formulas/mathematics/college/qr4s8t8mhw1uvr41wtor0em19ia4e5aaob.png)
If he races x=39 times, your friend would pay:
![y=7(39)+14=287](https://img.qammunity.org/2021/formulas/mathematics/college/oucf7g1uvxmgx9hyijvyuhn3j0jn1w0xq1.png)
y=287
He would pay $287 if he races 39 times