Question 1000151
You have to know how to read this.
The left side says:
e^[ some exponent ]
The exponent that {{{ e }}} is raised to reads as:
" I am an exponent ( natural log ) that gives you
{{{ 4x }}} when the base is {{{ e }}} "
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You do have this exponent with the base {{{ e }}} 
on the left side, so the expression is {{{ 4x }}}
Then,
{{{ 4x = 16 }}}
{{{ x = 4 }}}
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A simpler example of this would be:
{{{ 10^( log( 100 )) = x }}}
What is {{{ x }}} ?
The left side says:
" I am {{{ 10 }}} raised to an exponent "
The exponent says:
" I am an exponent and if my base is {{{ 10 }}},
then I give you {{{ 100 }}} "
The base is {{{ 10 }}}, so the left side equals {{{ 100 }}}
{{{ x = 100 }}}