Let's call the tread depth T, and the distance driven d. T as a function of d gives us:

Note that the maximum tread depth is 8mm. Let's start with that:

T(d) = 8

However, it is stated that for every thousand miles driven, that number decreases by 0.12mm. This means that for every mile driven, the tread depth decreases by 0.00012mm. So our function becomes:

Now, to find the distance where the tread depth is 2mm we simply have to use our new equation.

Therefore:

Now we just solve for to get: