Question 21887
For the missing number, I would use the variable X.
So, your equation is this:
{{{6781+2124=2124+x}}}
Then simplify to make the equation look easier
{{{addition_cartoon(6781,2124)}}}
Then you get 
{{{8905=2124+x}}}
How do you find X now? 
Isolate the X by subtracting 2124 from both sides
{{{subtraction_cartoon(8905,2124)}}}
So X = {{{6781}}}
Hope this helps!