To solve this problem it is necessary to apply the concepts related to Slit Diffraction.
The expression for separation between fringes is given by,
[tex]d= \frac{\lambda D} {a}[/tex]
Where,
[tex]\lambda =[/tex] Wavelength
d = Separation between fringes
a = Slit width
D = Distance between the slits
Re-arrange to find [tex]\lambda[/tex], we have that
[tex]\lambda = \frac{da}{D}[/tex]
Replacing with our values we have
[tex]\lambda = \frac{(0.005575)(4*10^-5)}{0.3}[/tex]
[tex]\lambda = 743.15*10^{-9}[/tex]
[tex]\lambda = 743.14nm[/tex]
Therefore the wavelength of the laser you used to collect the data is 743.14nm