Question

lcm between y+1 and y-3

Answer 1

To find the lcm, you want to simplefy your expressions to the maximum, and once this is done, you find the common factor of both and this is your lcm.

In your case, your expressions are has simple has they could be. So, in fact, you can write them as:
1*(y+1)
1*(y-3)
Which implies that the lcm is simply 1.

If you'd had, say:
y²+y and
y+1,
the first expression can be simplify as:
1*y*(y+1)
and the second is simply:
1*(y+1)

Your gcd would be y+1, but the least (if you include scalar(number)), is still 1. In the case we are really looking at a non-scalar lcm (you should look at the definition your teacher gave you or a textbook), it would also be y+1.

Is that clear?