\(-4x^2-24xy-36y^2\)
\(=-\left(4x^2+24xy+36y^2\right)\)
\(=-\left[\left(2x\right)^2+24xy+\left(6y\right)^2\right]\)
\(=-\left[\left(2x\right)^2+2\cdot2x\cdot6y+\left(6y\right)^2\right]\)
\(=-\left(2x+6y\right)^2\)
\(=-\left[2\left(x+3y\right)\right]^2\)
\(=-4\left(x+3y\right)^2\)