$If{\displaystyle \underset{k=1}{\overset{}{}}}k(\frac{2}{3}{)}^k=6,$ then what does

${\displaystyle \underset{k=1}{\overset{}{}}}(10k-3)(\frac{2}{3}{)}^k-{\displaystyle \underset{k=1}{\overset{}{}}}(5k-3)(\frac{2}{3}{)}^k$ equal?

Select one:

$a.12$

$b.18$

$c.24$

$d.30$