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We are given a quadratic function:

\[
f(x) = ax^2 + 4x + c
\]

The problem provides the following information:
- The vertex of the parabola is at \((h, k)\), and the graph opens upward, indicating that \(a > 0\).
- \( k < 0 \), meaning the vertex lies below the x-axis.
- We are also told that \( f(-9) = f(3) \).

### Step-by-step analysis:

1. **Quadratic Symmetry**: Since \( f(-9) = f(3) \), and the function is quadratic, the points \(x = -9\) and \(x = 3\) are equidistant from the vertex. This gives us the x-coordinate of the vertex:
   \[
   \frac{-9 + 3}{2} = \frac{-6}{2} = -3.
   \]
   So, the vertex is at \( h = -3 \).

2. **Vertex form of a quadratic**: The equation for the quadratic can be expressed in vertex form:
   \[
   f(x) = a(x + 3)^2 + k.
   \]
   From this, \(h = -3\) and \(k\) is the y-coordinate of the vertex.

3. **Using the standard form \( f(x) = ax^2 + 4x + c \)**:
   We also know that:
   \[
   f(x) = ax^2 + 4x + c.
   \]
   We can complete the square on this to convert it into vertex form. First, rewrite the quadratic part:
   \[
   f(x) = a(x^2 + \frac{4}{a}x) + c.
   \]
   Now complete the square inside the parentheses:
   \[
   f(x) = a \left( \left( x + \frac{2}{a} \right)^2 - \frac{4}{a^2} \right) + c.
   \]
   Simplify this expression to obtain the vertex form. However, since we already know the vertex coordinates, we can directly use them to determine further relations between \(a\), \(c\), and \(k\).

Now, based on the condition that \(k < 0\) and the symmetry in the equation, we need to explore which of the possible conclusions can be drawn. Would you like to know more details on this? Let me know if you'd like help solving for \(a\), or more steps to find the specific properties.

### Follow-up questions:
1. What is the formula for the vertex of a quadratic function in terms of \(a\), \(b\), and \(c\)?
2. How does the value of \(a\) affect the shape of a parabola?
3. What information can be deduced from knowing \(f(-9) = f(3)\)?
4. How do you complete the square to convert a quadratic function into vertex form?
5. What would change if \(k\) were positive instead of negative?

### Tip: 
Always use the symmetry property of quadratic functions when two function values are equal for different \(x\)-values. This can help find the vertex quickly.

In the given quadratic function, a and c are constants. The graph of y = f(x) in the xy-plane is a parabola that opens upward and has a vertex at the point (h, k), where h and k are constants. If k < 0 and f(−9) = f(3), which of the following must be true?

Analyze the quadratic function f(x) = ax^2 + 4x + c. Learn about parabolas, vertices, and the symmetry of quadratic functions. Suitable for high school students.

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