My quickest way is to write out eh addresses in binary. Then you can compare digits from left to right to find the longest match, which then becomes your supernet. For octects that are the same you don't have to write them all out.
Example:
a) 192.168.32.0/24
b) 192.168.41.0/24
c) 192.168.63.0/24
d) 192.168.75.0/25
What is the longest mask that can be used to summarize a,b & c? all four?
all are 192.168.x.0/24 so lets jest look at 3rd octect.
a) 0010 0000
b) 0010 1001
c) 0011 1111
d) 0100 1001
a,b & c all match the first 3 so a 192.168.32.0/19 summarizes all three
all four only match on the first bit, so 192.168.0.0/17 is the longest summary.
HTH
