Question 75916
substitution means taking a quantity from one equation and substituting its equivalent into another equation


In your case, taking the y from the first equation and substituting into the second equation gives x=3(3x+1)+1...every "y" in the second equation is replaced by its equivalent from the first equation "3x+1"... so x=9x+3+1...or x=-1/2


noting the symmetry of the two equations (they are identical, with the variables switched) means that y will also be -1/2.  You could confirm this by practicing a substitution from the second equation to the first