I think you're referring to the formula
[tex]v+f-e=2[/tex]
After all, we can't solve an equation for a variable if there's no equation to begin with!
With those assumptions made, add e to both sides:
[tex]v+f=e+2[/tex]
And subtract 2 from both sides:
[tex]v+f-2=e[/tex]
And we have solved the formula for e!
