Based on the way the polygons are marked, A corresponds with N, B corresponds with M, C corresponds with L and D corresponds with K. Using this, we set up a proportion:
12/8 = (5x+1)/(5x-11)
Cross multiplying, we have: 12(5x-11) = 8(5x+1)
Using the distributive property, 12*5x - 12*11 = 8*5x + 8*1 60x - 132 = 40x + 8