\[42x² - 150x + 125 = 0\ \ |\ :5\]
\[9x^{2} - 30x + 25 = 0\]
\[(3x - 5)^{2} = 0\]
\[3x - 5 = 0\]
\[3x = 5\]
\[x = \frac{5}{3}.\]
\[45x^{2} - 150x + 125 =\]
\[= 45 \cdot \left( x - \frac{5}{3} \right)^{2} = 5 \cdot (3x - 5)^{2}.\]