Question: the point-slope form of a line is y-y1=m(x-x1)?
Answer: The formula y - y1 = m(x - x1) is usually described as the 'point-slope form' for the equation of a line. It is useful because if you know one point on a certain line and the slope of that certain line, then you can define the line with this type of formula and, thus, find all the other points on that certain line.