The vertex of a function is controlled by the translations of the x and y value. Moving a vertex up and down is connected to the vertical position or the y-value. The y-value is the value that is added to the variable and is not squared.
Vertex form of a function is . In this case, k controls the vertical position of the vertex. If k is negative then the vertex will be moved down. So, if you want to move a vertex down by 5 units, then the k value must be -5.
Plug this into the formula to get the equation for k(x). This means that k(x) must equal .