Solution 3.10c
From Mechanics
The tension in the light string is the same all along the string. Thus the string exerts a force \displaystyle T on the block as in the figure.
\displaystyle \begin{align} & \to \ T-F=0 \\ & \text{Therefore}\ T=F=78 \textrm{.}4\ \text{N} \\ \end{align}