D'
you can't use the same single letter to mark a new point.
The square root of 81 is 9



which means either

or

. The equation has no solution, since

is always bounded between -1 and 1. The second has one solution at

, and any number of complete revolutions will also satisfy this equation, so in general the solution would be

where

is any integer.
So you could choose

and

.