(a)
\(a=2\,m / s ^2\)
\(g=9.8\,m / s ^2\)
\(Friction force = \mu g\)
\(=\mu \times 10 \times 9.8\)
\(=98\,\mu\)
Net force \(=\) External force \(=\) friction force
\(m \times a =40-98\,\mu\)
\(2 \times 10 =40-98\,\mu\)
\(98 \mu =40-20\)
\(=20\)
\(\mu=\frac{20}{98}=\frac{10}{49}=0.20\)