d
\(\begin{array}{l}
Equation\,of\,motion\,when\,the\,mass\,slides\\
down\\
Mg\sin \theta  - f = Ma\\
 \Rightarrow 10 - f = 6\\
\,\,\,\,\,\,\,\,\,\,\,\left( {M = 2kg,a = 3m/{s^2},\theta  = {{30}^ \circ }given} \right)\\
\therefore f = 4N
\end{array}\)

\(\begin{array}{l}
Equation\,of\,motion\,when\,the\,block\,is\\
pushed\,up\\
Let\,the\,external\,\\
force\,required\,to\\
take\,the\,block\,up\,the\\
plan\,with\,same\\
acceleration\,be\,F\\
F - Mg\,\sin \theta  - f = Ma\\
 \Rightarrow F - 10 - 4 = 6\\
F = 20N
\end{array}\)