how i get sqrt to biginteger number in c++

You might use the GMP library[^].

Here my stab at porting the algorithm from Solution 2 to BigInteger.

using namespace System;
using namespace System::Numerics;
...
        static BigInteger SQRT(BigInteger y)
        {
            if (y <= BigInteger::Zero)
            {
                if (y != BigInteger::Zero)
                {
                    return BigInteger::Negate(BigInteger::One);
                }
                return BigInteger::Zero;
            }

            /* select a good starting value using binary logarithms: */
            BigInteger x_old(4);
            BigInteger testy(16);   // testy = x_old ^ 2

            while(true)
            {
                if ( y <= testy)
                {
                    break;
                }
                testy <<= 2L;
                x_old <<= 1L;
            }
            /* x_old >= sqrt(y) */
            /* use the Babylonian method to arrive at the integer square root: */
            BigInteger x_new;
            while(true)
            {
                x_new = (BigInteger::Add( (y / x_old), x_old ) ) / 2L;
                if (x_old <= x_new)
                {
                    break;
                }
                x_old = x_new;
            }
            return x_old;
        }

Hope that helps.

There are a lot of integer sqrt implementations, as a first step you should search for one that contains only very simple operations on integers - operations that you can easily implement for big integers (like addition, shift, ...). For example this site contains a few integer sqrt algorithms: http://www.codecodex.com/wiki/Calculate_an_integer_square_root[^]
Choose an algorithm and write it with big integers instead of normal integers using your own biginteger library. Writing a bigint library is fun if you have time and interest. Of course you can use a third party biginteger library if it's allowed.