Answer:
[tex]\mathsf {y = -5}[/tex]
Step-by-step explanation:
[tex]\textsf {Given :}\\\textsf {-3x + 9y = -57}[/tex]
[tex]\textsf {Divide throughout by 3 :}[/tex]
[tex]\mathsf {\frac{1}{3} \times (-3x + 9y) = \frac{1}{3} \times -57}[/tex]
[tex]\mathsf {-x + 3y = -19}[/tex]
[tex]\textsf {Substitute x = 4 :}[/tex]
[tex]\mathsf {-(4) + 3y = -19}[/tex]
[tex]\mathsf {3y = -15}[/tex]
[tex]\textsf {Divide by 3 on both sides :}[/tex]
[tex]\mathsf {\frac{3y}{3} = \frac{-15}{3} }[/tex]
[tex]\mathsf {y = -5}[/tex]
