AAS stands for Angle-Angle-Side. Here, you are given two angles and one side that is not included in the angles that are corresponding to the angles and side of another triangle, so they are congruent by the AAS rule.
You may be asking why the ASA (angle-side-angle) is not applicable. This is because the side in the ASA rule should be between the two angles, in this case, the 10-unit-side is not in between the two angles but rather nearer to the angle 70°, so ASA is not the right answer.
