Question 881391
Use {{{x}}},{{{y}}} notation.
{{{y=x^2-4}}}
Interchange x and y and solve for y.
This new y is the inverse. 
{{{x=y^2-4}}}
{{{y^2=x+4}}}
{{{y=0 +- sqrt(x+4)}}}
In order for y to be a function, {{{x+4>=0}}} since the square root is only defined for non-negative values.
{{{x+4>=0}}}
{{{x>=-4}}}