Explanation:

3x+5y=9 …(1)
5x+3y=7 …(2)
Let us simplify the equations by eliminating a variable
Make the coefficients of X or Y equal
Multiply equation (1) with 5 and equation (2) with 3
We get 15x+25y=45 and 15x+9y=21
Now subtract the equations, we get
(15x+25y)-(15x+9y)=(45–21)
16y=24, y= 24/16 = 3/2
Substituting the value of y in equation (1) we get
3x+5(3/2)=9
3x+15/2=9
3x=9–15/2
3x=3/2
x =1/2
x+y =3/2+1/2= 2
x+y=2
The other method is simply add equations (1) & (2)
You get
8x+8y=16
x+y=2