f(x) = 3(1/2)x is not properly formed; if you meant exponentiation here, please write the function as
f(x) = f(x) = 3(1/2)^x.
The range is the set of all possible values of y (that is, all possible values of f(x). Try out a few x values and see what you get:
f(-5) = 3(1/2)^5 (note that this is positive)
f(0) = 3(1/2)^0 = 3 (this is positive)
f(4) = 3(1/2)^4 = 3/16 (this is positive)
There is no x value for which f is negative, and none for which f is 0. Given this info, what do YOU think the range of f(x) = 3(1/2)^x is?
Write it in the form (a,b).