First draw a diagram. The support is a rectangle defined by 2<x<2.5 and 2<y<2.3. Therefore the joint pdf is f(x, y) = 20/3 (why?).

Now note that if the two bids are within 0.1 of each other, then the region of interest lies between the lines y = x - 0.1 and y = x + 0.1.

The answer can be found using simple geometry - no need for calculus.