how do you differentiate

y = x/ ( a^2 - x^2) ^(1/2) - arcsin (x-a)

i tried to differentiate and i got

y' = a^2 / ( a^2- x^2) ^(3/2) - 1/ ( (1-(x-a)^2)^(1/2)

when the answer should be x^2 / ( a^2- x^2) ^(3/2)

may i know how i can continue from where i am stuck at?

thanks