Unfortunately, there's no easy way to post mathematical formulas here.
The possible number of addresses in a network is given by 2 to the power of the number of bits that can vary. Through algebra, you can show that the number of bits needed for a given number of addresses is the base 2 logarithm of the number of bits. This is sometimes abbreviated to 'lg'.
. That should give you the answer of nine, which is how many bits have to vary.
You then set the last 9 bits of a 32bit sequence to zero:
Split it into 4 groups of 8 bits:
11111111 11111111 11111110 00000000
Convert each byte into decimal:
255 255 254 0
And dot it: