Answer :
We have the system of equations:
[tex]\begin{gathered} -4x-3y=-21 \\ x=y \end{gathered}[/tex]We have to solve it by substitution.
The second equation gives the information to substitute, for example, x with y. We perfomr the substitution in the first equation:
[tex]\begin{gathered} -4y-3y=-21 \\ -7y=-21 \\ y=\frac{21}{7} \\ y=3 \end{gathered}[/tex]Then, with the value of y and the second equation we have:
[tex]x=y=3[/tex]Answer: the solution to the system is x=3 and y=3.