Vector Proof, need help.

[Isuckatphysics]

prove mathematically that for any two vectors u and w (bold = arrows on top) lying in the xy plane of a given reference frame

mag^2(u + w) = mag^2(u) + mag^2(w) +2mag(u)mag(w)cosq

where cosq is the angle between the directions of u and w

I just need help setting up the problem, because I have no idea where to begin.

[Tide]

Use the distributive property to evaluate
\left| \vec a + \vec b \right|^2 = (\vec a + \vec b) \cdot (\vec a + \vec b)