This is a Fraunhofer single slit experiment, where the light passing through the slit produces an interference pattern on the screen, and where the dark bands (minima of diffraction) are located at a distance of
from the center of the pattern. In the formula, m is the order of the minimum,
the wavelenght, 
the distance of the screen from the slit and 
the width of the slit.
In our problem, the distance of the first-order band (m=1) is
. The distance of the screen is D=86 cm while the wavelength is 
. Using these data and re-arranging the formula, we can find a, the width of the slit:
from the center of the pattern. In the formula, m is the order of the minimum,
In our problem, the distance of the first-order band (m=1) is