Answer: lol
Step-by-step explanation:
it's pretty simple. It's all about visualization.
So, take 2 and put it alongside its mirror image. <mirror image of 2><original 2>
That should look somewhat like a fish with a tail.
Now take 3 and put it alongside its mirror image. <mirror image of 3><original 3>
That looks like the number 8.
Same goes for 7. <mirror image of 7><original 7>
That looks like an inverted triangle, a triangle nonetheless.
I hope that explains the puzzle.