Can you please help me solve this problem.
A physics professor wants to perform a lecture demonstration of Young's double-slit experiment for her class using the 633- light from a He-Ne laser. Because the lecture hall is very large, the interference pattern will be projected on a wall that is 7.0 from the slits. For easy viewing by all students in the class, the professor wants the distance between the and maxima to be 20. What slit separation is required in order to produce the desired interference pattern?
Express your answer to two significant figures and include the appropriate units.