Answer:
396 
Step-by-step explanation:
Okay, so first we will find the triangle area.
9 * 8 = 72/2 = 36.
Now multiply this by 11 (the width of the prism)
36 * 11 = 396
It's very simple just divide 21.75 by five and that's your answer did you want me to do it?
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.
