1/3 c = {1/3 * (-3); 1/3 * 6} = {-1; 2}.
b = {-1 - 2; 2 - (-2)} = {-3; 4}.
Длина вектора b = sqrt((-3)^2 + 4^2) = sqrt(9 + 16) = sqrt(25) = 5.