Do you even Alge-bruh?

$\sqrt{\dfrac {\log_4{8}}{\log_8{4}}} = \sqrt{\dfrac {\frac{\log_2{8}}{\log_2{4}}}{\frac{\log_2{4}}{\log_2{8}}}} = \dfrac {\log_2{8}}{\log_2{4}} = \dfrac {3}{2} \quad \Rightarrow a = 3 \quad \Rightarrow b = 2$

$x^2 + Cx+D=21 \quad \Rightarrow x^2+Cx+D-21 = 0$ and since $a=3$ and $b=2$ are roots of the equation, then:

$\begin{cases} -C = a+b =3+2 & \Rightarrow C = -5 \\ D-21 = ab = 6 & \Rightarrow D = 27 \end{cases} \Rightarrow |CD| = 5\times 27 = \boxed{135}$

The result of division logarithm is equal 3/2 . We know that D-21=6 And we get D=27 And C=5 . Then D*C=135 .