Corresponding parts of similar triangles are proportional. The area of a triangle is 1/2bh. So DEF’s height is.
6 = 0.5(4h) 12 = 4h h = 3
We know, therefore, that the ratio of ABC to DEF’s dimensions is 12:4, 3:1. With this, we can deduce that ABC’s height is 9 ( 3 x 3), so ABC’s area is: