Double integrals question - please help (First Year Uni Maths)

Hi I'm really struggling with this double integrals maths question (below) and I was wondering if any keen mathematicians could give me some guidance.

Basically I'm not sure what to put as the limits for the integration when integrating with respect to y and x. The domain, D, is defined as x² + y² ≤ a², rather than it being described explicitly to me as x is between 'this number' and 'this number', so I'm not sure what to do.

Question:

Evaluate the double integrals over region, disk D, defined by x² + y² ≤ a².

a) ∫ ( 1 / √(x²+y²) ) dA