To find the value of 2n, we must first find what n is by itself. Given that n is f(3), we must plug in 3 into the f(x) function, and to every x in the function.
f(3) = (12 - 3)^3/2 (I wasn't sure if the 3 was multiplied to the (12-3) or if it was the power, but I assumed it was the power in this case.)
f(3) = 9^3/2 = 729/3 = 243.
Now that we have the value of f(3), we can say that n is 243. But since we need the value of 2n, we just need to multiply 2 to 243, giving us 486 as our answer.