Answer
$-15y^{2}+6y-4$
Work Step by Step
$(-6y^{2}+3y-4)-(9y^{2}-3y)$
First, remove the grouping symbols. To remove the second parentheses, change the sign of every term inside it:
$(-6y^{2}+3y-4)-(9y^{2}-3y)=-6y^{2}+3y-4-9y^{2}+3y=...$
Simplify by combining like terms:
$...=-(6+9)y^{2}+(3+3)y-4=-15y^{2}+6y-4$
