To determine how many people attended Leslie's birthday party using the given linear function model \( b(x) = 100 + 15x \), where \( x \) represents the number of people attending the party, we start by setting the total cost equal to the bill received.
Leslie received a bill of $550, so we can set up the equation:
\[ b(x) = 550 \]
Substituting the function into the equation gives:
\[ 100 + 15x = 550 \]
Now, we'll solve for \( x \):
- Subtract 100 from both sides:
\[ 15x = 550 - 100 \] \[ 15x = 450 \]
- Now, divide both sides by 15:
\[ x = \frac{450}{15} \] \[ x = 30 \]
Thus, there were 30 people in total at the party. However, since the question specifies "In addition to Leslie and Steve," we need to subtract the 2 known attendees from the total:
\[ \text{Number of other people} = x - 2 = 30 - 2 = 28 \]
Therefore, the number of other people who attended the birthday party is 28.