PreCalculus help please!

How do I transform this equation from polynomial form to vertex form by completing the square?!
f(x)=x^2-8x=21

Question

PreCalculus help please!


How do I transform this equation from polynomial form to vertex form by completing the square?! 
f(x)=x^2-8x=21

*insert name here* · Answer

You got the first step down, now do:
$$\LARGE (\frac{b}{2})^2$$

anonymous · Answer

see your question please.

anonymous · Answer

Okay, so I get 16?

anonymous · Answer

if the eq is $$x ^{2}+ax=b,then add \left( \frac{ a }{ 2 } \right)^{2} \to both sides.$$

*insert name here* · Answer

I'm gonna wait and see what @surjithayer has to say

*insert name here* · Answer

So once you get 16, just like surj is showing, you add it to both sides so:
$$\LARGE (x^2-8x+16)=21+16$$
Now put it in:
$$\LARGE (x \pm b)^2=c$$

anonymous · Answer

$$\left( x-4 \right)^{2}=37$$

*insert name here* · Answer

@surjithayer maybe we should let her do the work >.>

anonymous · Answer

Alright! Thank you guys, I think I get it now. I appreciate it! 
And I'm going over the work anyway @*Insert Name Here* , so don't worry. 
Thanks again! ♥

*insert name here* · Answer

Alright, good luck!