Suppose the wavelength of the light is 450 nm . how much farther is it from the dot on the screen in the center of fringe e to the left slit than it is from the dot to the right slit?

Since you are looking at a right fringe, the answer is all the time a multiple of the wavelength for the reason that the light makes constructive intrusion. If you are looking at the center (zeroth) fringe, the difference is (450)(0) = 0. For the first maximum, the difference is 450 nm. For the second, it is 450 x 2 = 900 nm. Basically, for the nth maxima, the difference is 450n.