1-3x-y=3
2x+y=25
Let’s start with the first one.
Subtract 1 from each side
-3x-y=2
Then add -3x to each side
-y=3x+2
Then move the negative and change the signs
Y=-3x-2
Now plug into the second equation
2x+(-3x-2)= 25
Now solve for x
2x-3x-2=25
Add 2 to both sides
2x-3x=27
Now subtract
-x=27
Move the negative to the other side because x can not be negative
X=-27
Now plug that into the second equation
2(-27)+y=25
Now just solve
-54+y=25
Y=79