Given the point (5, 6) and the slope of 5, find y when x = 21.
The slope of a straight line is given by the ratio of the difference between the y-coords and the difference between the x-coords.
i.e. slope = ( y2 - y1) / (x2 - x1)
The first point is (x1, y1) = (5, 6)
the 2nd point id (x2, y2) = (21, Y)
where Y is the value we seek.
So.
slope = 5 = ( y2 - y1) / (x2 - x1) = (Y - 6) / (21 - 5)
5 = (Y - 6) / 16
80 = Y - 6
Y = 86