we must match the imaginary part and the real part separately
SO
[tex]\begin{gathered} 3x=12 \\ -21=7y \end{gathered}[/tex]now, we can solve x and y
[tex]\begin{gathered} x=\frac{12}{3} \\ x=4 \end{gathered}[/tex][tex]\begin{gathered} y=\frac{-21}{7} \\ y=-3 \end{gathered}[/tex]check:
[tex]\begin{gathered} 3(4)-21i=12+(7(-3))i \\ 12-21i=12+(-21i) \\ 12-21i=12-21i \end{gathered}[/tex]right
