Since you have not mentioned any numbers to find the difference, I will assume the numbers on my own to be 6 and (-9).
To find the difference of these two numbers, you will need 6 white tiles and 9 black tiles.
Now to subtract (-9) from 6, you need the 9 black tiles to be replaced with 9 white tiles and change the subtraction sign to addition. Adding them will give you a result of 15 which will be white tiles.
6 - (-9) = 6 + 9 = 15
Answer: 15 white tiles