There are three ways in which negative numbers can be expressed in binary:
1. Sign magnitude form
2. One's complement form
3. Two's complement form
Out of these Two's complement method is most popular and is used in modern computers. In this method negative of a number x is represented by taking two's complement of the number. The given number is represented in 2's complement representation. If MSB of number is 1, it is negative number. Since in our example MSB is 1, so the number is negative number and we can find magnitude of number by taking its 2's complement
I hope you know how to find 2's complement of a number.
So, when you take 2's complement of 11010, it will be 00110 which is binary equivalent of 6. So given binary string represents -6