2d8+(STRx2)+(enhancement bonus x2)+(1d6 flaming).
Multiplying: When you are asked to apply more than one multiplier to a roll, the multipliers are not multiplied by one another. Instead, you combine them into a single multiplier, with each extra multiple adding 1 less than its value to the first multiple. For example, if you are asked to apply a ×2 multiplier twice, the result would be ×3, not ×4.
Multiplying Damage: Sometimes you multiply damage by some factor, such as on a critical hit. Roll the damage (with all modifiers) multiple times and total the results.
Note: When you multiply damage more than once, each multiplier works off the original, unmultiplied damage. So if you are asked to double the damage twice, the end result is three times the normal damage.
Exception: Extra damage dice over and above a weapon's normal damage are never multiplied.
You won't ever multiply the Flaming damage for a charge or crit.
If you charged and got a critical with a lance, which is a X3 crit weapon, you'd do
4d8+(STRx4)+(enhancement bonus x4)+(1d6 flaming) NOT 6d8+(STRx6)+(enhancement bonus x6)+(1d6 flaming)
If you managed to charge and crit a White Dragon or other fire vulnerable critter, your damage would be
4d8+(STRx4)+(enhancement bonus x4)+(1d6 flaming + 50% of the flaming damage's die result, rounded down)
If you have spirited charge and get a crit with a lance charge, you'd deal...
5d8+(STRx5)+(enhancement bonus x5)+(1d6 flaming)
And that is why everything will be trying to kill your mount and why the rules often make it difficult to charge.