f(x) = x^2 - 4
<em><u>Replace f(x) with y</u></em>
y = x^2 - 4
<em><u>Switch x and y.</u></em>
x = y^2 - 4
<em><u>Solve for y.</u></em>
<em><u>Add 4 to both sides.</u></em>
x + 4 = y^2
<em><u>√ both sides.</u></em>
√(x + 4) = y
<em><u>Switch sides, and replace y with f^-1(x)</u></em>
<u>f^-1(x) = √(x + 4)</u>