Question 1167405
{{{y=x^2-6x}}}
{{{y=x^2-6x+9-9}}}
{{{y=(x-3)^2-9}}}


{{{(y-3)^2-9=x}}}
{{{(y-3)^2=x+9}}}
{{{y-3=0+- sqrt(x+9)}}}
{{{y=3+- sqrt(x+9)}}}
Here you want y less than or equal to 3, and x must be greater or equal to 0.
Pick {{{f^(-1)(x)=3-sqrt(x+9)}}}.