y = x² - 18x + 30 in vertex form?

y = x² - 18x + 30

y = [x² - 2(9)x] + 30

y = [x² - 2(9)x + 9² - 9²] + 30

y = [x² - 18x + 9²] - 81 + 30

Vertex form :
y = (x - 9)² - 51

vertex form: y = a(x-h)²+k, where (h,k) is the vertex.

You need to know how to complete the square.
y = (x² - 18x) + 30
 coefficient of the x term: -18
 divide it in half: -9
 square it: (-9)² = 9²
 use 9² to complete the square:
y = (x² - 18x + 9²) - 9² + 30   note that you have to add 9² then subtract 9², to keep the equation balanced
 = (x-9)² - 51
vertex: (9,-51)