Olympiad Geometry · Medium problem

Triangles $ABC$ and $DEF$ are congruent by SAS, with $AB = DE$, $AC = DF$, and $\angle A \cong \angle D$. If $AB = 4x - 3$, $DE = 21$, $AC = 15$, and the perimeter of $\triangle ABC$ is $45$, find $BC$.