Watch the 2 GRID POINTS, at the
start and at the end of the line segment.
+ going 5 grids to the right means
+ going down 15 grids, so
+ going to the right 1 grid, means
+ going down 15 : 5 = 3 grids.
+ so the y coordinate of F is 45 - 3 = 42 and
+ point F has coordinates: (1, 42)