A system to 10 balls each of mass 2kg are connected via massless...

Acceleration,$a = \dfrac{Netpullingforce}{Totalmass}$

$a = \dfrac{6mg}{10m} = \dfrac{6g}{10} = 6m{s}^{- 2}$

Considering ball $8, 9 \text{ and } 10$ as system, tension between ball 7 & 8 can be written as,

${T}_{7, 8} = \left(3m\right) \times a$

$= 3 \times 2 \times 6 = 36N$