You can't substract the sqare root from a negative number. 
Always the result of a sqare root will be positive, cause will be in module |a|. And the result of the module is ALWAYS a positive number.

SO, the answer to you quesiton is "is's not possilbe" so substract the sqare root of -64.
You can substract the sqare root from 64 and the result is 8, cause 64=8x8, or 8^2.
Irs not possible bcoz it is acomplex no. and the topic complex no. is not in ur syallbus..
when u will come  in higher classes you will be learned about complex no..
so the ans. I NP...