Number multiplied by itself is same as the square of the number so the simplest way to find the number would be to take a square root of the given number.
√12345678987654321 = 111,111,111.
Hint: If your calculator is not equipped to work with large number you might see the not be able to type the entire number into it. The solution to this is just type the keyword “square root of (the number)” and google will display the answer for you. You can use this method to take square root of any large number.