x=17. You know the triangles are congruent, which means similar. That means that all the measurements of the triangles are the same. Because of this, you can set the 2 equations to equal each other. 3x+4=72-x Here you need to get all the x's on 1 side and the numbers on the other. You would add an x to each side and get 4x+4=72, then you would subtract a 4 from each side and get 4x=68. From here you just divide 68 by 4 and get 17.
