I think this should read f(x)=6(1.5)^x. In words this function means increase the length of the photo (6 inches) by 50% x times. That is, one enlargement of the photo is an increase in its dimensions of 50% or a half, so x enlargements gives a final enlargement of 1.5^x. When x=4 this comes to 5.0625, so its length is 6*5.0625=30.375 inches. This enlargement calculation is similar to calculating compound interest.