SAS (Side-Angle-Side) Postulate
If two sides and the included angle of a triangle are congruent to two sides and the included angle of another triangle, then the two triangles are congruent.
In the figure, 
.
Therefore, by the SAS postulate, 
.
If two sides and the included angle of a triangle are congruent to two sides and the included angle of another triangle, then the two triangles are congruent.
In the figure, .
Therefore, by the SAS postulate, .