
\begin{enumerate}
\item $\begin{aligned}[t]
A & =(2x-5)(x+3)+(x+6)^2\\
&=(2x^2+6x-5x-15)+(x^2+2\times6\times x+6^2)\\
&=2x^2+x-15+x^2+12x+36\\
\Aboxed{A&=3x^2+13x+21}
\end{aligned}$

\item $\begin{aligned}[t]
B & =(3x+4)^2-(x-1)(x+1)\\
&=(3x)^2+2\times4\times3x+4^2-(x^2-1^2)\\
& = 9x^2+24x+16-x^2+1\\
\Aboxed{B&=8x^2+24x+17}
\end{aligned}$

\item $\begin{aligned}[t]
C & =(5x-4)(5x+4)-2(x+3)(x-3)\\
& = (5x)^2-4^2-2(x^2-3^2)\\
&=25x^2-16-2x^2+18\\
\Aboxed{C&=23x^2+2}
\end{aligned}$

\item $\begin{aligned}[t]
D & =(8x+3)(3x-8)-3(x+9)^2\\
& = 24x^2-64x+9x-24-3(x^2+2\times9\times x+9^2)\\
& = 24x^2-55x-24-3(x^2+18x+81)\\
& = 24x^2-55x-24-3x^2-54x-243\\
\Aboxed{D & = 21x^2-109x-267}
\end{aligned}$

\item $\begin{aligned}[t]
E& =(7x-1)(7x+1)-5(3x-2)^2\\
& = (7x)^2-1^2-5\big[(3x)^2-2\times2\times3x+2^2\big]\\
& = 49x^2-1-5(9x^2-12x+4)\\
& = 49x^2-1-45x^2+60x-20\\
\Aboxed{E & = 4x^2+60x-21}
\end{aligned}$

\item $\begin{aligned}[t]
F & =3(2x-1)^2+(5x-3)^2\\
  & = 3(4x^2-4x+1)+(25x^2-30x+9)\\
  & = 12x^2-12x+3+25x^2-30x+9\\
\Aboxed{F & = 37x^2-42x+12}
\end{aligned}$
\end{enumerate}
