Solve using the substitution method

-x-3y=-20
5x+3y=40

-x=-20+3y

x=20-3y
5(20-3y)+3y=40
100-15y+3y=40
100-12y=40
-12y=-60
y=5

-x-3(5)=-20
-x-15=-20
-x=-5
x=5

check
-5-3(5)=-20
-20=-20

5(5)+3(5)=40
25+15=40
40=40