To find whether your solutions are extraneous or not, you need to plug each of them back in to your given equation and see if they work.

The solution you got for '​x*(4/3) - 5 = 11' is 'x=12'.

Plug 'x=12' into '​x*(4/3) - 5 = 11' and you will get an identity:

12*(4/3) - 5 = 11

16 - 5 = 11

11 = 11

So, 'x = 12' is not extraneous solution.