If Tana = - 2, then the square of Sina - 3 * (the square of COSA) =?

If Tana = - 2, then the square of Sina - 3 * (the square of COSA) =?

sin²a-3cos²a=(sin²a-3cos²a)/(sin²a+cos²a)
=(tan²a-3)/(tan²a+1)
=1/5