what you do is use y=mx+b formula. so use delta y and delta y/
You would do 3 over 6 that can be rudeced to so it qould be 1/2. that would be m. because thats the constant rate of change.
so you have y=1/2x+b so far now yuo have to find b so slove for b. what you would do first is put the 2 coordinates in place of x and y so you have
3=1/2(6)+b than you would have 3=3+b now subtarct 3 on both sides and you have 0=b. SO your fianlly answer would be y=1/2x. Hope this helps