\(\left( {{\text{W}}\,\, = \,\,\Delta {\text{KE}}} \right)\,\, - \,\,\mu mgx\,\, - \,\,\frac{1}{2}\,\,k{x^2}\,\, = \,\,0\,\, - \,\,\frac{1}{2}\,m{V^2}\)
\( \Rightarrow \,\,{V^2}\,\, = \,\,\frac{{1.44}}{9}\,\, \Rightarrow \,\,V\, = \,\,\frac{{1.2}}{3}\,\, = \,\,0.4\,\, = \,\,\frac{4}{{10}}\,\, \Rightarrow \,\,N\,\, = \,\,4\)