The position of a maximum given by interference created by a double slit is given by the formula:
d·sinα = m·λ
First, you need to find the angle α: you know the separation of the first two maxima and the distance of the slit from the screen, therefore, using trigonometry after transforming the units of measurements, you get:
y = 0.018 m
D = 3.5 m
α = tanâ»Âč(0.018/3.5) = 0.2945°
Now, solve for d in the first equation (λ = 6.11Ă10â»â· m):
d = m·λ / sinα
  = 1 · 6.11Ă10â»â· / sin0.2945
  = 1.187Ă10â»âŽ m
Therefore, the separation between the slits is 1.2Ă10â»âŽ m.