Hello,

The answer is 3.0 cm. The solution is as follows,

min.
fsinθ=nλfsinθ=nλ
sinθ=nλ6sinθ=nλ6
n=2n=2
sinθ=2λ6=tanθ1=x1Dsinθ=2λ6=tanθ1=x1D
x=4x=4
sinθ2=4λ6=x2Dsinθ2=4λ6=x2D
x2−x1=4λ6−2λ6=2λ6=2λ6=3cmx2−x1=4λ6−2λ6=2λ6=2λ6=3cm
width of central max = 2λ6=3cm

Hope this helps, Good Luck.