What's the wavelength of the light? You need that to find the answer.
Things to consider: What happens to the wavelength of light when it passes from air (n = 1) to glass (n =1.51)? Then ask, when light travels a distance L, what happens to it's phase? What's the difference in phase for the light in air versus the light in the glass? (Count the wavelengths!) For them to be out of phase, the number of wavelengths must differ by [tex](n+\frac{1}{2})[/tex] (where n is an integer >= 0); for the minimum thickness, choose n = 0.
