To find the cost of purchasing \( x \) books after applying the discounts, we need to subtract the discount function \( g(x) \) from the cost function \( f(x) \).
The functions are given as follows:
- \( f(x) = 15x + 8 \) (the original cost)
- \( g(x) = 2x + 5 \) (the discounts)
To find the total cost after discounts, we can express it as: \[ \text{Total Cost after Discounts} = f(x) - g(x) \]
Now we calculate this: \[ f(x) - g(x) = (15x + 8) - (2x + 5) \]
Now, simplify the expression: \[ = 15x + 8 - 2x - 5 \] \[ = (15x - 2x) + (8 - 5) \] \[ = 13x + 3 \]
Thus, the function that represents the cost of purchasing \( x \) books after the discounts is: \[ \boxed{13x + 3} \]