Answer :
to solve 3x-4 we need to know the value of x and we can do it from the equation 2x+1=3
then solve x to this equation
[tex]\begin{gathered} 2x+1=3 \\ 2x=3-1 \\ 2x=2 \end{gathered}[/tex][tex]\begin{gathered} 2x=2 \\ x=\frac{2}{2}=1 \end{gathered}[/tex]then the value of x is 1 and we can replace on 3x-4 to have the solution
[tex]\begin{gathered} 3x-4 \\ 3(1)-4 \\ 3-4 \\ -1 \end{gathered}[/tex]the value of the expresion is -1