Then continue as follows:-

For $m^{th}$ principal maximum,

$(a + b) sin θ = mλ → sin θ = \frac{mλ}{a + b}$

For minimum intensity in single slit

$a sin θ = nλ → sin θ = \frac{nλ}{a}$

If the two conditions are satisfied simultaneously,

$\frac{mλ}{a + b} = \frac{nλ}{a}$ …