ok thanks. how did you know it was this?

so from here x=asin(t) ....

then dx=a*-cos(t) dt still not exactly sure why this is.

now i am trying to find the limits so when x=a ... t=90 so

∫ 90,-90 2* √[b^2 - ((b^2 *a^2sin(t)^2)/a^2) a*-cos(t) dt ]

i am not rally sure how to integrate this