Once you know the slope plug it in for m in y mx b. Different words same formula Teachers use different words for the y-coordinates and the the x-coordinates. It was changed to y -1x 12 so it will look like y mx b slope intercept form.
ALSO whenever you see an X by itself make sure you put a 1 in front of it.
The equation is useful when we know. It emphasizes the slope of the line and a point on the line that is not the y-intercept. 12 is the y intercept b and -1 is the slope m and X is just there because its supposed to be. It was changed to y -1x 12 so it will look like y mx b slope intercept form.