По теореме синусов :

[tex]\displaystyle\bf\\\frac{28}{Sin150^\circ}=2R\\\\\\2R=\frac{28}{Sin(180^\circ-30^\circ)} =\frac{28}{Sin30^\circ} =\frac{28}{0,5} =56\\\\\\R=56:2=28\\\\\\Otvet \ : \ R=28[/tex]