Hey there! I'm happy to help!
Let's call the hamburgers h and the cheeseburgers c.
h+c=234
c=2h
Let's plug this value of c into the first equation to solve for h.
h+2h=234
3h=234
Divide both sides by 3.
h=78
Therefore, 78 hamburgers were sold on Thursday.
Have a wonderful day! :D