Question 918125
It really depends on what f(x) is.
You can graph it and look for where it crosses the x-axis but that's an approximate method and may require a lot of iterations to window in on a solution. 
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Some functions allow for simple algebraic solutions, such as,
{{{f(x)=x+1}}}
So,
{{{f(x)=0}}}
{{{x+1=0}}}
{{{x=-1}}}
So, {{{f(-1)=0}}}
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Others are not so easy and require numerical methods to solve. 
{{{f(x)=x^2e^(-2x) -xe^(-2x) -6}}} being a recent example. 
Solve using numerical methods, where approximately,
{{{x=-0.75508}}}
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But you really need to know f(x) to know which way to proceed.