Find the inverse

Algebra Level pending

Consider\quad f:R { + }→[4,∞)\quad given\quad by\quad f(x)=x^{ 2 }+4,\quad if\quad f\quad is\quad invertible\quad then\quad find\quad f^{ -1 },where\quad R { + }\quad is\quad the\quad set\quad of\quad all\quad non-negative\quad real\quad numbers.

-\sqrt { x-4 } \pm \sqrt { x-4 } { x }^{ 2 }-4 \sqrt { x-4 }