Question 75214
If I have 
{{{x+4=8}}} which solves to
{{{x=4}}}

So the solution is x=4. Now you would substitute 4 into x (4 would take x's place since x is really a placeholder for a number) to show that it's true
{{{highlight(4)+4=8}}}
{{{4+4=8}}}
{{{8=8}}}It's true and it works. The same idea applies to variables as well.