Well, I cheated on this one. I used the free app tiger algebra and this is what they came up with. So check it out. I hope it works. (5/3,-1/15) Good luck
X= 12(.7)^x(exponent) x is the doll you are on. For example if your looking for the height of the height of the doll after the 12 in. one then x=2 and so on
