Do you undestand what "y is proportional to 3^x" means? Do you understand what "A is proportional to B" means in general?
This comes out of a calculus book chapter one identifying functions and mathematical models:
The question is access whether the given data sets reasonably support the stated proportionality assumption. Graph an appropriate scatterplot for your investigation and, if the proportionality assumption seems reasonable, estimate the constant of proportionality
a.) y is proportional to 3^x
in a chart
Y:5,15,45,135,405,1215,3645,10935
X:0,1,2,3,4,5,6,7