I think you take 12.01 and multiply by "h" the hours, and then 4.05 by "x" for the people signing up. So I think it is 12.01h+ 4.05a. Does that sound right? You probably know this better than me ;)
On a number line, the length of a segment is the absolute value of the difference of the coordinates. So, if point has coordinate and point has coordinate , we have