#### Answer

x ... $\ \ y=0\ \ $ ... horizontal

#### Work Step by Step

Book reference:
Characteristics of Exponential Functions of the Form $f(x)=b^{x}$
(below Figure 4.4)
6. The graph of $f(x)=b^{x} $ (exponential function with base b) approaches, but does not touch,the [ x ] -axis .
The x-axis, whose equation is [ $y=0$ ], is a [ horizontal ] asymptote.