To determine the error, we will find the vertex by ourselves. To do so, we will first express the quadratic function in vertex form, where and are either positive or negative constants. Let's compare the general formula for the vertex form with our equation. We can see that and The vertex of a quadratic function written in vertex form is the point Therefore, the vertex of this parabola is Let's now examine the given solution.
We see that the value of was found correctly. However, the interpretation of the vertex form is not correct. Based on our operations at the beginning, we can correct the solution as follows.