Well for #13:
a) you know that the relation between the model and the building is proportional so you can solve it using proportions.
12 9
---- = -----
36 x
x=27 cm
another method you could use is to see the relation between them. For 12 to get to 36 is by multiplying by 3 so that same rule is applied to all measures. (You can use this method for letter a and leave the proportions usage for letter b)
b)
12 3
---- = -----
36 x
x=9
~Hope this helps~ ^^