acceleration

[bungie77]

I have a problem concerning acceleration. I know it seems simple but I couldn't get it.

http://img508.imageshack.us/img508/13/carad1.th.jpg (http://img508.imageshack.us/my.php?image=carad1.jpg)

Imagine a car accelerates from point A tangentially given a(t) = (5t^-1/2) ms^-2. I am asked to find the magnitude of the car's total acceleration in the x-y plane when it is at point B.

I'm not sure how to really go about the problem. Normally given a problem like this I would just solve for the magnitude of acceleration using normal and tangential components but I am unsure where to go from here.

[bungie77]

Sorry that image is really poor quality. Point A is 100 metres above the beginning of the half circle (which has radius 200 metres) and point B is located directly at the bottom of the half circle (or 270 degrees).