Let the restaurant need to earn x dollars on Sunday, to average 1000$/day .
The average earning over the 3 days is [tex] \frac{1240+921+x}{3}= \frac{2161+x}{3} [/tex] dollars.
we equalize this expression to $1000:
[tex]\frac{2161+x}{3} =1000\\\\ 2161+x=3000\\\\x=3000-2161\\\\x=839[/tex] dollars
Thus the equation is : [tex]\frac{1240+921+x}{3}(\$/day)=1000(\$/day) [/tex]