Question 143307
The problem is already solved for y in the first equation. Put that into the second and solve...

ie 2x + 3y = 31
         ^substitute x+7 for y
  2x +  3(x+7)=31
 5x=10
  x=2

If x=2, y must be 9.

Check:
x+7-->2+7=9  yes
2x+3y=2(2)+3(9)=4+27=31  yes

And here is how this site's solver would do it:
*[invoke linear_substitution "x", "y", 1, -1, -7, 2, 3, 31]