Optics: Solving for Slit Size Missing Fringes

[Feodalherren]

Homework Statement

Laser light is projected into two small slits that are 1mm apart. The fringe pattern on the screen is missing the 4th bright fringe. That is, there is a bright center fringe and three more bright fringes to either side of the central bright fringe, but the 4th one is missing. For that matter, the 8th is missing too. What is the size of the slit(s)?

Homework Equations

asinΘ=mλ

The Attempt at a Solution

 

[Simon Bridge]

That's quite messy - it is difficult to see what you've done.
Please show your reasoning with your working.
The marking seems pretty straight forward - you have miscounted the dark fringes, and attempted to count the dark fringes of the wrong diffraction pattern.