domain means the numbers you can use (basically, all real numbers except for those that make undefined stuff, like dividing by zero or having square root of negative)
ok so
1/(x-3)
no sqare roots so
find what numbers make divion by 0
1/(x-3)
1/0=1/(x-3)
0=x-3
3=x
3 makes a division by 0
3 is not allowed
domain is all real numbers except for 0