We know that number inside the box is the product of numbers on the circles beside it.
Since 5 x 9 = 45, the number in the first circle will be 5.
Then the number in the second circle will be 11 because 5 x 11 = 55.
Now the number in the circles are 11 and 6.
So, the number in the box = 11 x 6 = 66