\(V=1.800.000\left(l\right)=1800m^3=S.h\Rightarrow S=\frac{V}{h}=60\left(m^2\right)\)

\(\frac{x\left(x+y\right)}{y\left(x+y\right)}=\frac{-12}{21}=\frac{-4}{7}=\frac{x}{y}\Rightarrow x=-\frac{4}{7}y\)

Thay vào pt (1) ta có: \(\frac{-4}{7}y\left(-\frac{4}{7}y+y\right)=-12\Leftrightarrow\frac{-12}{49}y^2=-12\Rightarrow y=\pm7\Rightarrow x=\mp4\)

Do \(x<0\Rightarrow x=-4\)

\(P=4a^2+4ab+4b^2-12a-12b+12=\left[\left(4a^2-12a+9\right)+2b\left(2a-3\right)+b^2\right]+3b^2-6b+12\\ =\left(2a+b-3\right)^2+3\left(b-1\right)^2+9\)