Variable solved for

x=y-3

x+3y=13         substituting the first equation for x into the second, we have

[ y - 3] + 3y = 13      simplify

4y - 3 = 13     add 3 to both sides

4y =   16      divide both sides by 4

y = 4   

And using the first equation,  x = 4 -  3   = 1