/sci/ - Science & Math

how do i take the gradient of:

[math]
f(\boldsymbol x) = \frac{1}{2} \| \boldsymbol A \boldsymbol x - \boldsymbol b \|_2^2
[/math]

?

i know it's supposed to look like pic related but i'm unclear on how to get to it. i know that we can rewrite as

[math]
f(\boldsymbol x) = \frac{1}{2} (\boldsymbol A \boldsymbol x - \boldsymbol b)^T \cdot (\boldsymbol A \boldsymbol x - \boldsymbol b)
[/math]

not sure how to get any closer though... when i work the multiplication out it doesn't seem like i'm on the right track.