Hi yall im new to this forum but im looking for some help.

Find all values of the variable for which the expression is undefined.

1) (4y)/( x- 3)

2) (4x + 12)/ (x^3 - 9x)

3) (7)/ (7 -x )

4) (2y)/ (y^2 - 6y - 7)

5) (7)/ (x^2 -4)

6)(4x)/ (2x - 1)

Can u please help or teach me how 2 do it