x^2 - 8x + q = 0

D = 64 - 4q

x = (8 +- sqrt(64 - 4q)) / 2

(8 + sqrt(64 - 4q)) / 2 = (8 - sqrt(64 - 4q)) / 2 * 3   // *2

sqrt(64 - 4q) + 8 = 24 - 3 * sqrt(64 - 4q)   // - 8 + 3 * sqrt(64 - 4q)

4sqrt(64 - 4q) = 16

sqrt(64 - 4q) = 4

64 - 4q = 16

4q = 80

q = 20