Explain it if you know that 7x8=56,how can you use the commutative(order) property of multiplication to find the product of 8x7?

The communicative property for multiplication states that the order of the multiplicands and can be swapped and the resulting product stays the same.
So 
A * B = B * A 
So if
7*8=56
then the commutative property says
8*7=56 as well since you can swap the order of the terms on the left and not change the result.
This property is true for addition and multiplication. It does not hold for subtraction or division. 
Hope that makes sense and helps. (:

38+105= 143, 39+104=143, and you will find then adding the numbers on opposite sides, moving inwards yields the answer 138 every time. because of this, you’re going to multiply 138 times the number of pairs, which is 34. therefore, 34*138= 4692(i’m sorry if it’s the wrong numbers, but it’s the right concept)