Step-by-step explanation: I would first create a factor tree for these numbers and break them down into their prime factorization.
First, 28 is 7 x 4 and 4 is 2 x 2
Next, 35 is 7 x 5
In our factors tree, a 7 is a factor that is shared so we pull out a 7.
Now, we multiply all of our factors that don't pair up
and in this case, that's a 2, another 2, and a 5.
So our LCM is 7 x 2 x 2 x 5 or 140.