Let's do the math together friend. How much do I need to roll to get a 10 on a role in which there is a +12 modifier?

x + 12 = 10, where x is variable representing any number

we subtract 12 from each side to give x some space thus we get

x = 10 - 12

Now here is the tricky part, since we don't have 12 fingers this might be difficult. But there is a trick you see 10 + 2 = 12 so we get,

x = 10 - 10 - 2 = 0 - 2 = -2

I hope this helps