The triangles are similar. Specifically the largest triangle and the smaller triangle on the right side
We can form the proportion below and solve for x x/(x+3) = (x+3)/12 12*x = (x+3)(x+3) 12x = x^2+6x+9 x^2+6x+9 = 12x x^2+6x+9-12x = 0 x^2-6x+9 = 0 (x-3)^2 = 0 x-3 = 0 x = 3
