The given equation is
[tex]4x-3y=21[/tex]First, we subtract 4x from each side
[tex]\begin{gathered} 4x-4x-3y=21-4x \\ -3y=21-4x \end{gathered}[/tex]Then, we divide the equation by -3
[tex]\begin{gathered} \frac{-3y}{-3}=\frac{21-4x}{-3} \\ y=-7+\frac{4x}{3} \end{gathered}[/tex]