Question

Rewrite the equation in vertex form. y = x2 - 6x + 18

Answer 1

Complete square!
Vertex form is (x-h)^2+k
meaning that the vertex is located at (h,k).

$y=x^2-6x+18$
$=(x^2-2(3x)+9)+9$   3^2=9
$=(x-3)^2+9$   
vertex form, h=3, k=9, vertex is located at (3,9)

Edited 2018-03-18
There was a suggestion to explain the completion of square as
x^2-6x=(x-3)^2-9.

However, this does not show where the (x-3) come from.
If this is a better explanation for any or all of you, here it is.

Answer 2

The answer is y= (x-3)^2 + 9

I hope this helps