General conceopts:

a/b=x/y

a/b is the scale factor.

x/a is the image

y/b is the actual picture.

to find B: (y/x)*a=b ex: 1/b= 6/18      18/6=3       3/1=3       b=3

to find x: a*(y/b)=x  ex:  1/3=x/18     18/3=6      6*1=6    x=6

to find y: b*(x/a)=y ex:  1/3=6/y     6/1=6     6*3=18     y=18