What's the value of 4 square root of 81 to the 3rd power

I must assume that you meant [ 4sqrt (81) ]^3.

If that's the case, then you end up with 64 [ 9 ]^3 = 64 (729) = 46656.

You could remove ambiguity here by using parentheses. Note how I used parentheses (brackets) to convey my assumptions about what you were asking.