[tex]\bf \begin{cases}
y=x^2+2x\\
y=3x+20
\end{cases}\implies \stackrel{y~~ =~~ y}{x^2+2x=3x+20}
\\\\\\
x^2-1x-20=0\quad
\begin{cases}
-5+4=-1\\
-5\cdot 4=-20
\end{cases}\implies (x-5)(x+4)=0
\\\\\\
\begin{cases}
x-5=0\implies &x=5\\
x+4=0\implies &x=-4
\end{cases}[/tex]
[tex]\bf \stackrel{\textit{what is \underline{y} when x = 5? let's use the 1st equation}}{y=(5)^2+2(5)\implies y=25+10\implies 35}\qquad \qquad\qquad (5,35)
\\\\\\
\stackrel{\textit{what is \underline{y} when x = -4? let's use the 2nd equation}}{y=3(-4)+20\implies y=-12+20\implies y=8}\qquad \qquad (-4,8)[/tex]
