Hi there!
I'm just gonna answer this and explain as I'm doing it.
So we have (3x10)x8= __x(10x8)
Well, first we look at what they did. They couldn't have changed the equation, but just moved things around.
Now, assuming you know your properties, I'm gonna go ahead and answer it.
(3x10)x8=3x(10x8)
^^
I used the Associative property, which basically says that when 3 or more #s are multiplied, the answer doesn't change no matter how you group the numbers. See how they just moved the numbers around, and only swapped the 3 for the 8?
Hope this helped!