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.
Step-by-step explanation: All he did was took the amount of red and gave it a ratio of 7:1 meaning that he will increase the amount of red and make it 21 because the number 8 increased by 3 so 7 must increase by 3 as well.