Vehicle A is 25km due east of vehicle B. A moves due west at 60km/h and B moves due north at 80km/h. Find an expression for the distance of separation for the two vehicles t hours later. After how many minutes is this separation distance a minimum and what is this minimum distance?

Im really just not sure how to find the expression, but i think i know how to find at how many minutes the separation distance is minimum (differentiate the expression then make f'(x) = 0. Right?)

I found the expression for distance as d = 25 + 85x, but this is wrong. Apparently (according to my teacher) i need to use the Pythagoras theorem, so i guess that means im finding displacement and not distance.... now that i reread the question thats actually pretty obvious because its distance of separation not distance travelled. :/. oh well.

Anywho, any help in finding that expression is appreciated. Thanks.