\( 15 m^{4} n^{4}+6 m n^{3}+3 m^{2} n^{2} \)

Step 1 ：\(\displaystyle \text{Factor out } m\cdot n^{2} \text{ from the expression:} \)

Step 2 ：\(\displaystyle =m\cdot n^{2}(15m^{3}n^{2}+6n+3m)\)

Step 3 ：\(\displaystyle =m\cdot n^{2}(15m^{3}n^{2}+3\cdot 2n+3m)\)