A lot of different questions from a new player...

[Niwashi]

How do you know that a 15 level gear = to a level 17 gear? Is there a math formula to determine this?

It's called Item Level and is stated on the item's toolip (on the gray bar separating the top section with damage or defense from the next section with stat modifiers). All equippable gear has an item level. For the regular (light grey background) items, that Item Level will be the same as the level required to equip the item, but for some of the better (green or blue background) gear (certain dungeon drops while leveling, or a wide variety of endgame gear), the Item Level is higher to indicate it's better than the regular stuff you'd equip at that level.