It is true due to the commutative property of multiplication. If all of your operations are multiplication, then the integers can move around and you can still get the same answer.
^This is only true if you meant to type (1.8x3)x2.1=(1.8x2.1)x3
If you meant what you typed, then the equation is false. The left side would be 3 times the value of the right side due to the 3 being on the left side and not the right side while without the 3 the two sides would be equal.