And it's elimination using addition and subtraction
x-y=1
x+y=3

X-y=1. > x=1+y
(1+y)+y =3
1+y+y =3
1+2y =3
2y=3-1=2 >y =2/2=1
Y=1
X-y=1
X-1=1. >x=1+1 =2

X=2. Y=1

Get one x+y=3 to y=x+3 and plug it into x-y=1 and you get x-(x+3)=1. Destitute the x and solve. Once you get what x equals plug it to to either equation for the value of x and solve for y