Two ways to find the vertex that I can think of are:
Putting the eqn into turning pt form
or
Finding halfway between the x-intercepts if they exist
Which way suits you?
Completing the square is the same method as turning pt form.
First step is to tkae out half as a common factor
Now we need to change the constant term from inside the brackets so we can form a perfect square. To do this we take the co-efficient of the middle term, half it and square it. This gives +9 giving
Then making the perfect square complete.
Can you take it from here?