Question 756372
{{{ f(x) }}} says " the right side of this equation 
depends only on the value of {{{x}}}"
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And, if you look at the right side, it depends 
only on the value that you choose for {{{x}}}
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{{{ f(3) }}} tells you to choose {{{ x = 3 }}}
for the right side, so
{{{ f(3) = 3*3 - 7 }}}
{{{ f(3) = 9 - 7 }}}
{{{ f(3) = 2 }}}