An equation to represent this would be 12x+2=T.
x=10 people per large table.
If T=122 then we just have to isolate x on one side of the equation. The first step is to replace T with 120 and then subtract 2 on both sides (to cancel the +2) to get 12x=120. Cancel the 12 by dividing both sides by 12 to get x=10. The number of people seated at each large table would be 10. Hopefully I explained this correctly and helped you understand.