Please show all work and formulas. Please don't skip math steps

Problem 6: Doorway Diffraction. Sound of frequency f.:= 1250-11.: leaves a room through a LOU-m-wide doorway. At which angles relative to the oentedine perpendicular to the doorway will someone outside the room hear no sound? Use for the speed of sound in air and assume that the source and listener are both for enough from the doomay for Fraunhofer diffraction to apply. You can ignore effects of reections