Answer
$A = 90^\circ \hspace{10pt} C =60^\circ \hspace{10pt} c = 7.274 \,$ cm
Work Step by Step
$\sin{A} = \dfrac{a \sin{B}}{b} = \dfrac{8.4 \times \sin{30}}{4.2}$
$\sin{A} = 1$
$A = 90^\circ$
$C = 180-(A+B) = 60^\circ$
$c = \dfrac{b \sin{C}}{\sin{B}} = \dfrac{4.2 \times \sin{60} }{\sin{30}}$
$c = 7.274 \,$ cm