The yard space is large enough to have a playhouse that has a width of 3.5 meters. If Parker wants to keep the playhouse in proportion to the model, what cross multiplication of the proportion should he use to find the height?

 define
W= width playhouse real=3.5
H= height playhouse real

x=width playhouse model
y=height playhouse model
therefore

H/W=y/x------------------>H=y*W/x------------>H=3.5*y/x

The answer is 3.5*y/x to find the height H