How to Simplify tan (pi/2 - pi/11)

How do you simplify tan (pi/2 - pi/11)? I got 9pi/22 but my math hw online says it's not correct, help please? I think I might be missing a step. And no calculator use! Simplify to most you can without calculator

tan(pi/2 - pi/11)

tan([pi/2*]11 - [pi/11]*2)

tan( 11pi/22 - 2pi/22)

=tan ( 9pi/22 )