Which function has an inverse that is a function?

1. b(x) = x2 + 3
2. d(x) = –9
3. m(x) = –7x
4. p(x) = |x|

1. y = x^2 +3 
x = y^2 +3
x - 3 = y^2
y = + or - √(x-3) not a fction

2. y = 0x -9
x = 0y -9
x = -9 not a fction

3. y = -7x
x = - 7 y 
y = - x/7 yes function

4. y = |x|
x = |y|
y= x,-x not a function

Dis the solution here

Ithink the versus years?