You just put the scale of one building on one side and the scale of the similar building on other side, with width and length written in same place (denominator or numerator). With x for the part that you dont know
Length of one building / width of one building = length of other building / width of other building
12 / 9 = 100 / x
Solve for x, by cross-multiplying
12 * x = 100 * 9
12x = 900
X = 900/12
X = 75 ft
