Promise no more questions today! ^_^

The reason I'm stuck on these is because I've completely forgotten what im doing, after returning to these from months ago.

I need to:

1) Express $\displaystyle x^2 - 8x + 15$ in the form $\displaystyle (x + a )^2 + b$ where "a" and "b" are the numbers to be found.

This is what I've got thus far:

$\displaystyle y = x^2 - 8x + 15$

$\displaystyle = (x - 4 )^2 - (4)^2 + 15 = 0$

$\displaystyle (x - 4)^2 - 16 + 15 $

Then I need to plot for the original equation. Not alot