To determine the number of toppings you chose to put on your pizza, we can set the total cost equal to the function \( f(x) = 0.75x + 9 \) and solve for \( x \).
Given that the total cost is $13.50, we can set up the equation:
\[ 0.75x + 9 = 13.50 \]
Next, we'll isolate \( x \):
- Subtract 9 from both sides:
\[ 0.75x = 13.50 - 9 \] \[ 0.75x = 4.50 \]
- Now divide both sides by 0.75 to solve for \( x \):
\[ x = \frac{4.50}{0.75} \] \[ x = 6 \]
Thus, the number of toppings you chose to put on your pizza is 6.