Hello!
First we find the slope, which is the the difference of the y-values divided by the difference of the x-values as seen below.

The slope of our line is 1.
Now, the y-intercept is where x=0 is located at. As you can see, we already have a point with x=0. Therefore our y-intercept is 1, as the y-value is in that ordered pair.
Just to check, we will plug in a point from our line (1,2) in the slope intercept equation and solve for b.
2=1(1)+b
2=1+b
b=1
We can write our equation in standard form (Ax+By=C) below.
-x+y=1
I hope this helps!