we have the following equation:
We resolve for x, that is, imagine the following arrangement of arrows:
that is equivalent to say that the number 13 is multiplying the number 12 and the number 52 is multiplying the number x. So for resolve x, we have that the number that is multiplying to x ( the number 52) has to divide into our new equation, as follows:
that is equivalent to:
So, we can conclude that x = 3. Then, The blank space must be filled with the number 3.