ANSWER
This formula is used for finding the equation of a line when you have been given the slope of the line, m, and a point (x1, y1).
EXPLANATION
We have the equation:
y - y1 = m(x - x1)
We want to know what this formula is used for.
Generally, this formula is the most popular method of finding the equation of a line when you have been given the slope of the line, m, and a point (x1, y1).
Because x1 and y1 are numbers, and m is also a number (constant), the formula can be expanded to result in:
y = mx + c
m = slope
c= y intercept
That is:
y - y1 = m(x - x1)
y - y1 = mx - mx1
y = mx - mx1 + y1
y = mx + (y1 - mx1)
where c = y1 - mx1
This is possible because y1, x1 and m all have constant numerical values.
