$R(m=3)=mN=(3)(16000lines/in*2.5in)=120,000$

$R(m=2)=mN=(2)(16000lines/in*2.5in)=80,000$

$\Delta \lambda=\frac{\lambda}{R}=\frac{550nm}{80000}=0.006875nm=0.069 \overset \circ A$