Answer
a) $\theta_{iM}=\arcsin(\frac{v_i}{v_r})$
b) $0.44m$
Work Step by Step
a) $\frac{\sin(\theta_r)}{\sin(\theta_i)}=\frac{v_r}{v_i}$
$\theta_r=90^o$
$\theta_{iM}=\arcsin(\frac{v_i}{v_r})$
b) $v_i=343m/s$
$v_r=1440m/s$
$\theta_{iM}=\arcsin(\frac{343m/s}{1440m/s})=13.78^o$
$\tan(\theta)=\frac{x}{y}$
$x=y\tan(\theta)=(1.8m)\tan(13.78^o)=0.44m$