SOLUTION
[tex]\begin{gathered} Given \\ 4x-5y=21\text{ and 2x+5y=0} \end{gathered}[/tex][tex]\begin{gathered} Let \\ 4x-5y=21-----(1) \\ 2x+5y=0------(2) \end{gathered}[/tex][tex]Equation\text{ \lparen1\rparen plus equation \lparen2\rparen}[/tex][tex]\begin{gathered} 6x+0=21 \\ 6x=21 \\ x=\frac{21}{6} \\ x=\frac{7}{2} \end{gathered}[/tex][tex]plug\text{ x=}\frac{7}{2}\text{ into equation \lparen2\rparen to obtain y;}[/tex][tex]\begin{gathered} 2x+5y=0 \\ ie\text{ 2\lparen}\frac{7}{2}\text{\rparen+5y=0} \\ 7+5y=0 \\ 5y=-7 \\ y=-\frac{7}{5} \end{gathered}[/tex]Final answer:
[tex]\begin{gathered} x=\frac{7}{2} \\ y=-\frac{7}{5} \end{gathered}[/tex]