Answer
$3.94m$
Work Step by Step
We know that
$\theta=tan^{-1}(\frac{y}{L})$
$\theta=tan^{-1}(\frac{0.765m}{2.10})=20.01^{\circ}$
$\frac{\lambda}{d}=sin\theta$
$\frac{\lambda}{d}=sin(20.01^{\circ})=0.342$
For the second order $m=2$
$sin\theta=2(0.342)$
$\implies \theta=sin^{-1}[2(0.342)]$
$\theta=43.15^{\circ}$
Now, the linear distance for the second order maximum can be determined as
$2y=2Ltan\theta$
$\implies 2y=2(2.10m)tan(43.15^{\circ})=3.94m$
