So you have the distance in terms of two functions in t and you can plug those functions of t into your distance formula, then you derive it and plug in the value of 2 mins. (keep in mind that your rate is in hours and your t value is in minutes so I converted 90 km/h to 3/2 km/min by dividing by 60 and similarly converting 80km/h to 4/3 km/min.

I guess and easier approach would be to plug in before deriving so you have:

distance =

then simplifying you get: distance =

Derive that with respect to t to get:

D(distance) =

I derived that using first the power rule

and the chain rule:

Then plug in your value of t = 2 min to get the answer.