Which is the correct way to evaluate f(15) for the function f(x)=2(x+3)?

start by substituting 15 for x. then, simplify to get f(15)=36.
since f(15) is not within the domain of the function, it cannot be evaluated.
since f(15) means the output value is 15, the function evaluates to f(15)=15.
start by letting 2(x+3)=15, then solve for x. when solved, the answer is x=4.5.