Why does it say invalid argument? sqrt(11, 0)

I don't get what you want your argument to be.

If you are taking the square root of a coordinate point, you have do individually root it for the x argument and the y argument.

If you mean 11.0, just type 11. 

In general, your system is interpreting your answer as the sqaure root of a coordinate point, which is obviously invalid.

What you have entered    sqrt ( 11,0)      is    this :   \(\sqrt[0]{11}\)

   which would be     11^1/0       and we all know you cannot divide by 0