Triangles MNC and RSC are similar because all 3 angles are the same, so the the ratio between each side should be the same.
If NC=12 and SC=6 then triangle MNC is scaled up by 2 from triangle RSC. This means each side of MNC is 2 times bigger than on RSC. So, sense RS=4, we can multiply that by 2 to get MN=8.
