Question 777454
A) Picture the X-Y graph. There are an infinite number of possible straight lines. If we fix the slope, the lines are all parallel, but there are still an infinite number. But, once the slope is fixed, and you tell me any point, only one line can pass through it. 

The form is Y=mX+b where m is the slope and b is the Y-intercept. Don't be fooled. The Y-intercept is simply a point with the distinction that X=0. 

B) If you tell me two points, I know that only one straight line can pass through these two points, and the equation becomes m=(y2-y1)/(x2-x1) to solve for the slope, and then choose either point, y-y1=m(x-x1) to fill in the equation. 

To determine the equation of a straight line, you need either two points or one point and the slope.