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

[Zilon]

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

[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.

[ErnestoGiovanni]

no it will say on it "Item level #" The text is in Japanese yes, but "ITEM LEVEL 100" is clear, that is where you look, the level that it is equipable is lower on the display

[ErnestoGiovanni]

Look at this picture too ther green text tells you what level you can equip it and right above that it says what level the item's stats are.