[tex]\bf \begin{cases}
t=tacos\\
m=milk
\end{cases}~\hspace{7em}
\begin{cases}
\stackrel{1~taco}{t}+\stackrel{1~glass}{m}=\stackrel{\$}{2.10}\\[0.8em]
\stackrel{2~tacos}{2t}+\stackrel{3~glasses}{3m}=\stackrel{\$}{5.15}
\end{cases}
\\\\[-0.35em]
\rule{34em}{0.25pt}\\\\
\begin{array}{llll}
t+m=2.10&\stackrel{multiplied~by}{\times -3}\implies &\stackrel{notice}{-3t\stackrel{\downarrow }{-3m}=-6.3}\\[0.8em]
2t+3m=5.15&\implies &~~2t+3m=5.15\\
\cline{3-3}
&&-t+0m=-1.15
\end{array}
\\\\[-0.35em]
~\dotfill[/tex]
[tex]\bf -t=-1.15\implies t=\cfrac{-1.15}{-1}\implies \blacktriangleright t=1.15 \blacktriangleleft
\\\\\\
\stackrel{\textit{and now substituting on the 2nd equation}}{2(1.15)+3m=5.15}\implies 2.3+3m=5.15
\\\\\\
3m=2.85\implies m=\cfrac{2.85}{3}\implies \blacktriangleright m=0.95 \blacktriangleleft[/tex]
notice, we used -3 in the multiplication to eliminate the m's.
