algebra precalculus - Why does $k^2+4k+4$ become $(k+2)^2$?

I'm studying maths after a lot of years without having a look at it.

Currently I'm doing proof by induction, I have a doubt when doing an exercise.

If you have a look at this video at minute 5:15 , $[k^2+4k+4]$ becomes $(k+2)^2$ and I don't understand why.

Is there any rule or theorem that explains this?

Answer

depending on the area of rectangle

$$k^2+2k+2k+4=(k+2)(k+2)$$
$$(k^2+4k+4)=(k+2)^2$$