Because the reactant is a solid (n = 0) and there are 2 moles of gas in the product (n = 2).
? ?n = 2 – 0 = 2

The equation you're cited is easy to derive:
Kp = P(NH3) • P(HBr) = {n(NH3)RT/V} • {n(HBr)RT/V} = {n(NH3)/V}•{n(HBr)/V}•(RT)²
= [NH3][HBr]•(RT)² = Kc •(RT)²

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