How do you find the vertex of a parabola ... ?

Well you would have to use the formula x = - b/2a to find the value of x and then substitute that value of x in the equation to find y

The vertex formula is derived from the completing-the-square process, just as is the Quadratic Formula. In each case, memorization is probably simpler than completing the square.) For a given quadratic y = ax2 + bx + c, the vertex (h, k) is found by computing h = - b/2a, and then evaluating y at h to find k.