we can split the square into two right triangles. Becasue the perimeter of the square is 54cm, we know the each side is 13.5 cm (54cm / 4). Using the pythagoras theorem, the diagonal is 19.1 cm.
You first find its intercepts: the x intercept is (5/3,0) and the y intercept is (0,5) (just plot those points). Then make the line connecting those 2 points!
