x squared minus four, at which intervals is the function increasing or decreasing?
if f '( x ) > 0 then f is increasing
if f '( x ) < 0then f is decreasing
In this case:
f ( x ) = x ^ 2 - 4
f '( x ) = 2 x
f '( x ) < 0 on interval (-infinity , 0 ]
f '( x ) > 0 on interval [ 0 , infinity ]