Since you're only interested in x, you can use the first equation to write an expression for y that can be substituted into the second equation.
.. y = k -x
.. 2x +3(k -x) = k +1
.. -x +3k = k +1 . . . . . collect terms
.. 2k -1 -x = 0 . . . . . . subtract k+1
.. 2k -1 = x . . . . . . . . .add x
The 3rd selection is appropriate.
