Olympiad Geometry · Medium problem

Triangle $ABC$ is congruent to triangle $RST$, with correspondence $A \leftrightarrow R$, $B \leftrightarrow S$, $C \leftrightarrow T$. If $m\angle A = 3x^\circ$, $m\angle B = (x+10)^\circ$, and $m\angle C = 70^\circ$, find $m\angle R$.