For this case we have that by definition, the slope of a line is given by:
[tex]m = \frac {y_ {2} -y_ {1}} {x_ {2} -x_ {1}}[/tex]
We have as data that:
[tex]m = -1\\(x_ {1}, y_ {1}) :( 1,2)\\(x_ {2}, y_ {2}) :( 2, y)[/tex]
Substituting we have:
[tex]-1 = \frac {y-2} {2-1}\\-1 = \frac {y-2} {1}\\-1 = y-2\\-1 + 2 = y\\y = 1[/tex]
Finally, the value of y is 1.
Answer:
[tex]y = 1[/tex]
