Here you have to figure out what number each fruit represents. So considering the three equations on the left:
Cherry = 4
Apple=3
Watermelon=6
Now, look at the diagram and the 3 numbers on the right. The number below the smallest circle will be 4 since it represents the Cherry. The number below the largest circle is 6 since it represents the Watermelon. Similarly, the number below the medium circle will be 3. So the numbers from left to right will be 364. Input them to obtain the key and exit the level.