The vertex is -0.25
Explanation: I have a Casio fx-9750Gll. I pressed menu and clicked on graph. i then typed in the equation and pressed EXE(which is enter). then, i pressed f5(G-Solv) and that gives you two options, Max or Min. you can look at the graph to determine which one it is. but i pressed f3(V-Window) which was the option Min and it gave me (2.5,-0.25) so the answer is -0.25
