to subsitute you mean
make one side equal to only one placehloder
so -x+y=-5
make equal to y
add x to both sides
y=x-5
subsitute x-5 for y in second equation
3x-5(x-5)=9
3x-5x+25=9
add like terms
-2x+25=9
subtract 25 from both sides
-2x=-16
divide by 2
-x=-8
multiply -1
x=8
subsitute 8 for x in first equation
y=x-5
y=8-5
y=3
x=8
y=3
