domain is what you can put in
the domain for most functions is ALL REAL NUMBERs
except for those that violate the dividing by zero thingummy
example
if yu had
f(x)=x+2
domain is all real numbers
f(x)=2/x
domain is all real numbers except for 0 since cannot divide by 0
f(x)=2(x-4)
domain is all real numbers except for 4
so answer is {x|x is a real number}
