Finding the acceleration

Homework Statement

a small body is moving in the plane xy with a constant (in size) velocity [ sorry for my english]. Find the acceleration for x=0 for the parabola $$y=ax^2$$

The Attempt at a Solution

$$v^2= a_x^2 +a_y^2 \\ \vec{v}= a_x\vec{x}+ a_y*\vec{y}$$ and $$\vec{a}=d\vec{v}/dt$$

I'm kind of lost now, mainly because I'm new to differentiating vectors... Any help would be helpful...

Constant velocity means that acceleration is 0 or that the direction of velocity is changing, so is the velocity changing direction?

haruspex