21) Using the substitution method,
Notice that
[tex]\begin{gathered} y=y \\ \end{gathered}[/tex]
Then,
[tex]\begin{gathered} \Rightarrow4x-1=-2x-7 \\ \Rightarrow6x=-6 \\ \Rightarrow x=-1 \end{gathered}[/tex]
Finding its corresponding value of y,
[tex]\begin{gathered} x=-1 \\ \Rightarrow y=4(-1)-1=-5 \\ \Rightarrow(x,y)=(-1,-5) \end{gathered}[/tex]
Therefore, the answer is x=-1, y=-5
