When calculating the quadratic function in vertex form, you just have to create a quadratic binomial with the values in the function, and then you add or withdraw from the equation what you need to complete the quadratic binomial.
So you now know that to create a quadratic binomial, you have to divide the B by 2, and then square it, remember that "B" is the numeric value on the second term of the equation, in this case is -16, so divided by 2 it would be -8 and squared it would be 64.
So now you know that the quadratic function in vertex form is: