Double intergral using polar

i'm sorry if this look messy and uni computers does not have latex installed, kindly please help to convert it to latex if possible and thank you in advance

Evaluate the following interral using geometrical interpretation

I= intergral( intergral( (a^2 -x^2 -y^2)^1/2 dxdy , where R is the disk given by

R = {(x,y):x^2+y^2 (smaller or equal than) a^2

the next step that i have done is to change it to intergral( intergral( (a^2 -r^2)^1/2 r.dr.d(digt)

after that i'm lost can any kind soul enlighten me on further steps?

and thank in advance to the guy if he manage to change this question to lastex format.