Young's double-slit question! physics! pleas help!?

In a Young’s double-slit experiment, a set of parallel slits with a separation of 0.100 mm is illuminated by light having a wavelength of 589 nm and the interference pattern observed on a screen 4.00 m from the slits. (a) What is the difference in path lengths from each of the slits to the screen location of a third-order bright fringe? (b) What is the difference in path lengths from the two slits to the screen location of the third dark fringe away from the center of the pattern?