Two triangles have equal bases. The altitude of one triangle is 3 units more than its base while the altitude of the other is 3 units less than its base. If the areas of the triangles differ by 21 square units, find the altitude of the smaller triangle.

Two triangles have equal bases. The altitude of one triangle is 3 units more than its base while the altitude of the other is 3 units less than its base. If the areas of the triangles differ by 21 square units, find the altitude of the smaller triangle.