\[\left( 0,6 \cdot 5^{3} - 15 \right)^{2} = \left( \frac{3}{5} \cdot 125 - 15 \right)^{2} =\]
\[= (3 \cdot 25 - 15)^{2} = (75 - 15)^{2} = 60^{2} =\]
\[= 3600.\]