How do i figure these out?:

The expression log(base 2)*(x-4) is undefined for all values of x such that

(1) x>1 (2) x<or equal to 4 (3) x>0 (4) x<or equal to 0

The expression log(base 3)*(8-x) is defined for all values of x such that

(1) x>8 (2)x<8 (3)x>or equal to 8 (4)x<or equal to 8

All i need to know is how to do them, i have the answers.