The last one.
To invert a function, let's solve the equation for [tex] x [/tex]. You have:
[tex] y = \frac{2}{3}x \implies x = \frac{3}{2}y [/tex]
[tex] y = 9x+1 \implies x = \frac{y-1}{9} [/tex]
[tex] y = -3x+\frac{1}{2} \implies x = \frac{y-\frac{1}{2}}{-3} [/tex]
[tex] y = x^2-5 \implies x = \pm\sqrt{y+5} [/tex]
The first three options are polynomials, and thus functions.
The last one, given the [tex] \pm [/tex] sign, associates two different values of y for every x value, and thus it's not a function.
