The two triangles are similar triangles.
The smaller triangle has hypotenuse 6 and a horizontal leg of 4
The large triangle has hypotenuse 6+12=18 and a horizontal leg of x, where x is some number larger than 4
Because the triangles are similar, we can form the proportion below and solve for x
x/4 = 18/6
6*x = 4*18
6*x = 72
6*x/6 = 72/6
x = 12
Final Answer: 12
