Let w represent the width of the rectangle in cm. Then its length in cm is (3w+9). The perimeter is the sum of two lengths and two widths, so is ...
... 418 = 2(w + (3w+9))
... 209 = 4w +9 . . . . . . divide by 2, collect terms
... 200 = 4w . . . . . . . . subtract 9
... 50 = w . . . . . . . . . . divide by 4
... length = 3w+9 = 3·50 +9 = 159
The dimensions of this piece of land are 159 cm by 50 cm.