The question states that this point is on segment , with the distance between this point and being the distance between to .
It could be shown (using a pair of similar right triangles) that the ratio applies not only to the (sloped) distance between this point and , but to the vertical distance as well.
The vertical distance between this point and would also be the vertical distance between and .
The vertical distance between and is the difference between their coordinates, .
The vertical distance between this point and would be of the vertical distance between and , .
Since is above , any point on the segment between these two points would also be above . Add the vertical distance between and the requested point to the coordinate of to find the coordinate of the requested point: .
$25 is a fixed cost and will have to be paid if the plumber comes and $15 times the number of hours it takes to do the repair is the variable cost. 'x' in this case is the number of hours. So the equation will be 15x + 25 = y