In ∆ABC and ∆DEF, AB = DF and ∠A = ∠D. The two triangles will be congruent by SAS axiom if:
AC = DE
Two triangles will be congruent by SAS axiom if 2 sides and the included angle of one triangle are equal to the two sides and the included angle of the other triangle.
Thus, the triangles will be congruent when AC=DE.