To determine the number of toppings you chose to put on your pizza, we can use the given linear function \( f(x) = 0.75x + 9 \), where \( x \) represents the number of toppings.
Given that the total cost of the pizza was $13.50, we can set up the equation:
\[ f(x) = 13.50 \]
Substituting the function into this equation:
\[ 0.75x + 9 = 13.50 \]
Next, we'll solve for \( x \) by first isolating the term with \( x \):
- Subtract 9 from both sides:
\[ 0.75x = 13.50 - 9 \]
\[ 0.75x = 4.50 \]
- Now, divide both sides by 0.75:
\[ x = \frac{4.50}{0.75} \]
\[ x = 6 \]
Thus, the number of toppings you chose to put on your pizza is 6.