To solve the quadratic equation $ax^2 + bx + c = 0$, the value of $c$ is identified as -12 from the graph. The vertex can be calculated using the formulas for the x-coordinate ($-b/2a$) and the y-coordinate ($-D/4a$). By substituting the known values into these formulas, the solution can be derived. Understanding these concepts is crucial for mastering precalculus. Proper application of these formulas will lead to accurate answers.