Question 999394
The two brackets stand for the absolute value function.
It's a function that always returns a positive number or zero, never a negative number. 
The definition is, if the argument is positive, the value of the abs. value is the value of the argument.
If the argument is negative, the value of the abs. value is (-1) multiplied by the argument.
So example,
{{{abs(5)=5}}}
{{{abs(-18)=-(-18)=18}}}
Pretty straightforward.
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{{{abs(x-4)=2}}}
Since you don't know the value of the argument, you basically solve both problems.
Assuming it's positive,
{{{x-4=2}}}
{{{x=6}}}
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Assuming it's negative,
{{{-(x-4)=2}}}
{{{x-4=-2}}}
{{{x=2}}}
So those are the two solutions, {{{x=2}}} and {{{x=6}}}