\[3 \cdot (y + 5)^{2} - 3y^{2} =\]
\[= 3 \cdot \left( y^{2} + 10y + 25 \right) - 3y^{2} =\]
\[= 3y² + 30y + 75 - 3y^{2} = 30y + 75\]