how to find the residue of -2/ (3z^2 - 10iz -3) at z=i/3.

i know that this has poles at i/3 and 3i

so i used the formula of

-2 /( i/3- 3i) and got 3i/4 where the denominator is (z_ -3i)

but when i used

-2 /( 6z_ - 10i) and sub z_=i/3, i got the answer as -i/4. (the denominator here is done by differentiation)

i thought that both methods should give the same answer?