Puzzle ZWSM 

Workings

Hint
Look at the rows.

Answer
4.

Reasoning
For each row, the left minus the right gives a number.

The numbers for the rows are then multiplied together.

In the first box:

   7 - 3 = 4
   4 - 1 = 3

so 4 x 3 = 12

In the second box:

   9 - 2 = 7
   8 - 3 = 5

so 7 x 5 = 35

In the last box:

   6 - 2 = 4
   8 - 7 = 1

so 4 x 1 = 4.