The given system of equations is
[tex]4x-y=7\text{ take it as equation (1)}[/tex][tex]-12x+3y=-21\text{ take it as equation (2).}[/tex]
Consider equation (1)
[tex]4x-y=7[/tex]
Subtracting 4x from both sides, we get
[tex]4x-y-4x=7-4x[/tex]
[tex]-y=7-4x[/tex]
Multiply both sides by -1, we get
[tex]-y(-1)=(-1)7-(-1)4x[/tex]
[tex]y=4x-7[/tex]
Substituting y=4x-7 in equation (2), we get
[tex]-12x+3(4x-7)=-21[/tex]
[tex]-12x+3(4x-7)=-21[/tex]
