Question 130306
The slope is given by {{{(y[1]-y[2])/(x[1]-x[2])}}}.  So just substitute the coordinates from the two points.  It doesn't really matter, but I'm calling (1,y) {{{P[1]}}} and (2,4) {{{P[2]}}}.


{{{(y-4)/(1-2)}}}


{{{(y-4)/-1}}}


But we are given that the slope equals 1, so:


{{{(y-4)/-1=1}}}


{{{y-4=-1}}}


{{{y=3}}}