The system of equations is given to be:
[tex]\begin{gathered} 3x+4y=12 \\ 9x-4y=24 \end{gathered}[/tex]To eliminate y, we can add the two equations together:
[tex]3x+4y+9x-4y=12+24[/tex]Collecting like terms and solving, we have:
[tex]\begin{gathered} 12x=36 \\ x=\frac{36}{12} \\ x=3 \end{gathered}[/tex]The value of x is 3.
